The first thing would be to make sure the professor you would be working with does not expect anything else. That would mean speaking to people who know them, have worked with them, or even email some of their past PhD students to see what they are like as a supervisor. Even better if there is someone in the department you know well that you can ask about them (via unofficial channels as any bad habits are more likely to be glossed over via email ect). And when speaking to them about it, it is worth raising this as a concern and ask about it. If they try to emotionally manipulate you (e.g. 'you should do this because you love it' or something to similar effect) then you can just safely ignore the offer.

A practical point if you do start is to organise regular meetings with your supervisor (which you can also ask them about) and keep track of what you accomplished in the last week and what you aim to do in the next week and in the next month to help keep you on track. This may help you be more productive, but you are going to have to work smart during those 9-5 hours. You may also have to be flexible and expect to work longer some weeks and less other weeks, or working a non-standard work week (particularly if you have international collaborators or experiments to run).

But the most important question is why are you considering doing this PhD? You said that you would be happy not to complete. Is this a field that you are interested in and want to learn more about? Is this a field you want to work in at some point in the future or are open to an academic career? Are the skills you would pick up important for future work or would you like to do a start-up in this field? Is this a placeholder that you aren't against doing but really is just there to fill time?

If this is not important for your goal career or not something that you really want to learn more about (and are willing to dedicate a number of years of your life to) then you might want to ask yourself what other options could you take that would help you towards that goal. If you are interested in start-ups you could try to get employed at a start up to see how people run them, as well as potentially getting some connections that are important for initial funding. But also keep in mind that if you are interested in start ups in this field you have been working on then the skills you would gain in a PhD would be useful, particularly there are some similarities between a start-up's lean development, and the hypothesis design/testing of research (I'm particularly thinking in the sciences).

It is actually very difficult to maintain any boundaries while doing PhD. Up to degree that in some universities PhD students employed 50 % are expected to work somewhat 120 %, and whose who do not are quickly dismissed by professor simply saying "I think you are not the right person type for a scientist".

Researchers are, you see, mad a little bit, just like a pop culture pictures them. If you want to "balance life and work" or something the like, just do not join them.

Similarly, students are often expected to work hard and not to "balance life and work" instead. PhD is still considered an education.

I do not know, maybe somebody see this answer as disrespect to something but I would like bystanders to know how the knowledge and technology we all later enjoy is built. Regardless if you search for a new planet or a new algorithm of computing, it is always a real hell of work and uncertainty.

Same as with a related question, there is an issue of the field.

You might not be able to, if your field does not permit

• Are you basically doing an office job in a low-competitive field? Probably, yes.
• Are you literary working in a field? Probably, no.
• Are you working with animals or cell cultures? Probably, no. And also forget about holidays and weekends for the duration of your PhD.
• Is it easier / cheaper to get machine / instrument time during non-working hours and you really need those 1000000 cores / 100000 MWh / 1E14 MeV / 7 meter dishes for your research? Probably, no.
• Do the experiments take a lot of time and typically finish in the middle of the night? Probably, no.
• Do you need to perform observations at night, because Sun is evil? You guess it!
• It is well accepted in your field that a person cannot maintain top intellectual performance for the full 8 hours and you really need that to get your research done? Probably, yes. (However, my impression is that mathematicians use quite every single moment of time for their research, they just interweb highly demanding parts with routine.)
• Are there too many aspiring PhD candidates and too few positions? Probably, no. Because, why get someone, who works 40 hours/week, if you can get someone willing to work 80 hours/week for the same money? Sorry, this is cynical and probably against the law.
• Again, are there too many aspiring PhD candidates and too few positions? Then it would be possible to hire someone for 20 hours/week and to coerce them to work 40+ hours and also on a weekend. Again, labor laws might have a different opinion on that. But, probably, no.

What are your goals?

People here mentioned a lot about PhD studies not being a job, research passion, and so on. If some anecdotical evidence helps you, most people I know, who have remained in academia, do not work 9-to-5.

So, if your goal is to stay at the university after your PhD: probably, no.

But why?

You might ask, why are we doing all this? Why are many of us sacrificing a lot of non-working time to do work-related things? Very simple. Research, and, by extension, academia, is not a job. It's a passion, which, coincidently, gets the bills payed.

basically view it as a very poorly paying industry job while maintaining time for personal projects/startup ideas.

That is a bad way to start a PHD. There are a lot of "poorly paid industry jobs" in which the misalignment between you and your supervisors expectations will be smaller.

Yes, you can do a PHD 9 to 5, yes you may even get the title, and yes, maybe your supervisor is ok with the 9 to 5 part. What they definitely will be not ok with is that getting the phd for you is a low priority.

Maybe they would be happy to hire you as a lab technician (yes, bright people are needed there too, even if they don't want to get professor). But the continued mismatch between your goals and what you signed up for can not be a persistent thing. What I could imagine what works is: you check for 6 months, maybe you opinion changes. If it doesn't, you talk to you prof an tell him you are willing to work in the lab, but that a scientific career may not be for you (that is a discussion they will have had more often in their life).

It is an interesting question. What you did not mention is what discipline you want to do a PhD in and in what country.

For instance, in the UK, PhD costs a lot of money (about £9000 per year for at least 3 years for home students and at least twice that for internationals). So, if you are paying and have done 2 years already (and spent £18000 only on fees and about the same on living costs) you are unlikely to do out.

Saying that, most people do not pay for themselves though. There are a lot of funded positions (and you will even get reasonable bursary), but they have a catch. If you are funded by the department or university, you will have to do teaching (usually, 500 hours per year, lab sessions, marking, etc). Alternatively, your position can be funded through the project grant which your supervisor has. In that case you will have to work on the project besides your PhD topic. In any case, you will effectively have two jobs.

Also, discipline do matter. I cannot say for Humanities (probably reading enormous amount of literature), but on physics, chemistry, biology you will have at least some experimental work. Some experiments are very-very long and/or require attendance over several days (including weekends). You may have some resources they require regular maintenance, e.g. mice or mosquitoes which need to be fed and cared for 7 days a week. So, in this case keeping it to 40 hours a week is virtually impossible.

In disciplines like maths, computer science, data science etc it is a bit easier. You can choose when and where you are working, but these disciplines usually involve a lot of coding, which again usually requires about 10 times more time than planned (due to code debugging).

I have done PhD in applied math, I was definitely working much less than 40 hours a week (except about 2 months during writing thesis) and still did it in less than 3 years. Yes, I was also teaching and doing actually much more than 500 hours a year (getting very generous pay for the extra hours).

In general, I would not say that normal prof will demand you to work any specific number of hours. Academia is about flexibility. Nobody cares whether you are working 1 hour a day or 20 hours a day. All people care is the results you get.

The biggest issue here is that I am afraid, you have a bit wrong attitude towards academic work in general and PhD in particular. Even if you do not actively working on something, you usually keep thinking about something ("where is the mistake?", "How to make this work?", "How to improve this?", etc). Academia is not an industrial job, it is a way of life. And PhD is not different. And you should enjoy this way of life. At the same time I should admit that I can spend a lot of time with family and maintain healthy work-life balance.

So, if you are not sure, I would suggest going to industry for several years and then decide whether you need a PhD or not. Actually, many large companies would happily fund your PhD course if you will make a compelling case that you need it to fulfill your duties better.

You state clearly that you wish to have finite commitment because you want to reserve time for other projects. What you are describing is de facto part-time studies.

If you are serious about your PhD, it will require as much time as needed to complete it, with little to no time for start-ups. If you are serious about a start-up, this will also require as much time as needed, leaving no time for a PhD. [Famous quote: you do not own a start-up, the start-up owns you.]

In a PhD or in the start-up world, there will be moments when 40hrs/week will be vastly insufficient because of deadlines, i.e. exams, presentations, prototyping etc to prepare. If you are working on an experiment or doing field work, you need to do as much as possible when the apparatus works or when you are in the field: one does not stop at 5pm after spending all day correctly tuning some piece of equipment. If you have an investor meeting on a Monday morning, expect to spend the weekend working on your product, or the sales pitch, or whatever is required by your boss on Monday morning or before.

Finally, if you want your supervisor to commit time and resources towards your success, you better first show that you yourself are willing commit all the time needed.

I like your question. I don't think so what you propose is possible. Time limits would only be enforceable in jobs which are well defined for example cutting the grass, fielding sales calls, etc. PhD is an open-ended job, quite unlike conventional ones. It is done with a fixed goal in mind. The goal may be to publish a certain amount of papers, do sufficient research, etc. Basically do some amount of work which would satisfy your supervisor. When the works are ill-defined they are less enforceable in the amount of time they take. This problem is surely going to torpedo your working hour limits. Hence it would either cause you to leave within a month or so or you would continue to solder on having completely betrayed yourself.

Of-course all this is assuming you can get an advisor to agree with your terms of working hour limits. You should count yourself to be very lucky indeed to get hold of such an advisor in the first place.

Talk about this in your interview. If you aren’t having an interview, you need to make sure there is one - you need to hear the professors expectations, and share yours. If you aren’t on the same page with the professor about hours and other factors, you’ll have a challenge staying in the limits you want to set yourself, or it will lead to conflict.

A 40-hour per week doctorate is possible, if you are not expected to teach or pick up other duties besides those purely related to your research.

If you want to maintain this boundary, I would say it's quite simple. You tell the professor now, before starting the PhD: "I will not work beyond 5pm or on the weekends. I do want to keep side hustles going. Do you still want to offer me the PhD position?"

If they say yes, awesome, start the job. If they ask for more time from you, remind them of the boundaries you told them about before starting. As you say, there is not a whole lot that can be done to you, if getting the PhD or the recommendation is not important to you. However, do not be surprised if under these conditions, they do not want to offer it to you: I know of multiple professors who refused to take students who wanted to keep their own company going during the PhD. Precisely for the reason you mention: if the company takes off, the student will usually want to focus on that and not their PhD. And do be honest about this: pretending the PhD is your ultimate goal to the professor, while it's not, is not ok.

That said, even if the professor says yes, I would think really hard about whether you want to do this. There is nothing wrong with protecting your time off, and I actually think that on average I did not work much more than 40 hours per week during my PhD (I did some 80-hour work weeks before conference deadlines, but there was probably also enough slacking to offset the extremely busy weeks).

But this attitude of really not wanting to work a minute more and not caring (a whole lot) whether you finish is not often found in PhD students. It is also an attitude that may limit what you get from the PhD in terms of opportunities. If you're also happy to walk away after two years with no papers and no new opportunities (and possibly some burnt bridges) - then you have nothing to lose. If you would not be happy with that outcome, then consider it some more.

I do think it is possible to do a PhD and only work 40 hrs a week on a normal week, if you are head down, supper efficient and committed with in that time. I'm not sure how possible it is if you are absolutely ridged about that, never working more than 9-5 irrespective of the circumstances. There will be times when experiments take longer, or deadlines are approaching when more is needed. The better organised you and the supervisor are, the less common such time will be, but I don't think they can ever be completely eliminated. For what its worth, I think this is also true of any profession job, academic or not.

In terms of commitment, I think its healthy to feel that you could walk away if it doesn't work out. But do have to want it to work out. The way you word things makes me feel like it working out and you finishing are not even your best case scenario (which would be your side hussle to pay off). If you are taking a PhD, the supervisor (at least a good one) if investing a lot of their personal capital and work into you. A student who leaves is a black mark against a supervisor. For a young supervisor in a competitive field it can be career-ending. For a poor or abusive supervisor this is deserved. And if the PhD is making somebody unhappy, hopefully a good supervisor would be able to take it on the chin. But to go in with this being your preferred outcome is not a good sign. This is what I meant when I talked about taking a PhD "in good faith" on your previous question.

If what you want is a poorly paying industry job, get a poorly paid industry job. A PhD is not an equivalent experience.

"basically view [the PhD] as a very poorly paying industry job while maintaining time for personal projects/startup ideas"

Only that a PhD is not an industry job. In fact it is not even a job, at least not in most cases, or the successful cases.

How does the following sound like? "I want to star in films by Quarantine Tarantino or Martin Scorsese and become a Hollywood star. But I'm afraid they will push me to work beyond 40 hours per week for each film I'm starring in!"

Like any other highly creative and competitive work (e.g., film starring), doing research is not a job (again, in the successful cases). It is a dedication that one usually is passionate about. Viewing research and being an academic as a job is flawed in my opinion. Although it is possible to reduce it merely to "a job", it is logically flawed. If it is merely a job then it is not a good one: you can work less, be under much less pressure, and earn more and faster in other jobs.

Hence, my answer is that the premise of your question is dubious, and thus there seems not to be an appropriate answer to your question in the first place.

First of all, I know many PhD students (also myself) who did exactly that and finished their phd: They worked 40 hours a week (or less), had a "normal life" , knew they would go to industry afterwards and wanted to learn/do research before (and stay connected to the system "university") because they loved uni/studying. It helps that in my country, studying and also titles are traditionally seen as something valuable (so there is no feeling of "only study if this aids you in your future job" in my country). Some students also saw it as a fun experience to live abroad before returning home. For me, it was similar: I didn't want to become a researcher because for me the postdoc life seems horrible -- but one can do a phd realtively risk-free. (Now I teach at university).

It is certainly not possible to work only 40h with all profs/in all subjects. Maybe also not in all countries (in which one do you want to study?) Probably it is also not possible with the most famous universities/professors.

I do recommend you to do good research on your prof what kind of person they are. Is it possible to do this kind of work with them or not?

I do think your attitude "I am not dead set on completing" is great. If the prof makes unreasonable demands (or other things like misconduct), just go. When they suggest longer working hours, tell them you don't want to do this unless absolutely necessary, if they keep insisting, just go. Do keep your eyes open while doing the phd for skills you need in industry.

Note that there are even (incredible) people who finish a PhD and have little kids (and some of them, no partner!)

(Of course, you might have two fewer papers afterwards for a good university career, but as this doesn't seem to be your goal..)

One of the nice things about working at a university is that the working hours tend to be very flexible. In my experience, this is the main reason why PhD students (and faculty) don't have a 9 to 5 workday. Some work late, but many of those start late. Some work in burst, working long hours for some weeks (before a deadline), and taking it easy in other weeks. This flexibility is realy nice, but it does make it easier for advisors to demand unreasonable working hours from PhD students. We have all heard horror stories, but none of this happened to me or anyone I know directly. Most advisors are just normal humans who don't want to exploit others. Also, the topic of the power imbalance between advisors and PhD students is very well known in universities, and in all universities I have been at there are many faculty who may not be actively searching for signs of abuse, but do keep an eye open. None of this guarantees that no abuse happens, but it does put the horror stories in perspective (that does not help if you find yourself in such a horror story)

I have known one PhD student who maintained a Monday to Friday 9-5 workweek. This requires a lot of discipline, as you cannot rely on the institution to impose those hours on you (that is the flip-side of flexibility). She could do that by being very efficient while at work. By doing so, she got more work done than most PhD students who worked long hours. People, including her advisor, knew that, and respected her for that.

However, given the way you describe your "interest" in the position (not interested in completing, very poorly paid industry job), it does not seems you have the right motivation for this job. So I would recommend you think again whether this is really what you want to do.