If I have 10 BTC in total and zero fiat, how can I calculate how long can I "burn" my BTC for my survival?

But what then if the Bitcoin price doesn't go up, or even goes down? Or happens to be low each time I sell them on my schedule?

but I've determined that all "lending" options are either just not available to me, or way too risky.

I don't know how these statements match each other: you fear that relying only on BTC is too risky, but want to rely on it as everything else is too risky. (What are "lending options" in this statement?)

The only answer can be that you shouldn't rely solely on BTC or other cryptocurrencies. Period.

A sensible ratio depends on your personal risk level: you can be fine with a 5% allocation, or 20%, or even more. But if you want to bet your retirement on that, I'd be cautious.

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But I think that wasn't your question.

If we assume that you absolutely want to do this as described, and you want to know how much you can have every month, you have two options:

1. You divide your 10 BTC by 50 and then by 12, that's 0.01667 BTC/month. That's how much you can withdraw over 50 years. If the price is $40000, you get $667, if the price is $60000, you get $1000. The higher the price, the higher your monthly amount. But you never know how much money you get every month.

2. You take a fixed amount every month, as you said, e. g. $1000. That means you sell 0.0167 BTC when the price is $60000, you sell 0.025 BTC when the price is $40000, you sell 0.01 BTC when the price is $100000. You have a fixed amount every month, but you don't know how long your BTC will last. That's kind of dollar cost averaging in reverse.

   You can modify this logic by only selling if the price is "high enough", but how do you determine that?

For both kinds of scenarios, you'd need to predict the price of the BTC for the next 50 years. Maybe it will go up and eventually reach $1000000 (or so), or it will lose its importance over the decades and go back where it came from and eventually vanish. We don't know.

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Additional thought:

me sitting there in five decades, as an elderly man, still launching Bisq on some kind of Windows 10 OS on my dusty old hardware from 2019, to sell some coins to pay for my rent, food, electricity and Internet...

This thought is flawed, or I understand it wrong: if BTC is still relevant in 50 years, it will surely run on computers which are up to date at that era. If it isn't, and you'll be tied to hardware from 2019, you won't have anyone to sell it to and you're stuck with that useless bit combinations in your computer which were so hard to calculate.

Taking the BTC out of the equation, the normal way to deal with this kind of situation when people retire (and hence don't have any further expected income, but don't know how long they will live or what their elderly care needs / expenses will be) is to use your lump sum of assets to purchase an annuity. This is a product that will pay out a guaranteed amount every month for the remainder of your life.

The annuity provider will ask you a bunch of questions (do you smoke, did both your parents die young of heart attacks etc) and then determine how much they will pay you each month. Fair warning: for a lump sum of $625k (10 BTC) at the age of 35 the annuity rates you would get would be pathetic, nowhere NEAR the $1000 / month you would get if you did this yourself. Effectively the difference between the $1k and whatever the annuity provider offers you is you paying them to take on the risk for you - even if you live to be 130 they will have to keep paying out, whereas in your own personal plan, you're screwed if you live longer than 85.

The thing that is particularly unusual about your situation is that you are 35 and don't seem to expect to have any further income for the remainder of your life. If you're not able to work due to disabilities or caring responsibilities etc then there may be government benefits you can claim which would supplement your income. However, if you can work and can hence defer the time from when you need to be drawing down on your lump sum, that will make your retirement situation much rosier when you get there.

Just to make 100% clear: I am not recommending that you take out an annuity (and even if I was, advice you get on the internet from unqualified people is worth nothing). However, looking into the model that annuity providers use may help you in understanding better how to move forward with your own plans. There is more info about annuities here: https://money.usnews.com/investing/investing-101/articles/things-you-need-to-know-now-about-annuities (this is a USA-specific link - annuities are handled differently from a tax perspective etc around the world).

You are asking a financial modeling question, and as some others have pointed out, the investment vehicle isn't what's critical. What is critical are the assumptions that you make (as you implicitly acknowledged).

The core part of your question is, how much can I withdrawal from my investment so that the investment is not depleted until 50 years have passed?

This calculator will help you answer the question.

If you try it, you'll want to set the schedule type to "savings." You can then make an assumption as to how much the asset will appreciate each year (enter in the interest rate your assumed appreciation rate - which can be 0 or even negative) and then set up a "Withdrawal" series for an "Unknown" amount that can be increased (if you want) by a percentage. See "Percent Series" under cash flow options. The calculator will calculate the increasing withdrawal amount and create a schedule.

The calculator is not the easiest to use, but it will solve the problem. If you have questions about its use, leave them in the comment section at the bottom of the page.

Full disclosure, I created the calculator.