Is it appropriate to use a tautology (e.g. "period of time) in an essay for a college biology class?

I’ve been working with Grammarly for basic grammatical errors on my essay, and this is pushing me to the point of bafflement and frustration. I understand that it is a computer program and is therefore not perfect, nor will it fully understand the nuance of what I’m trying to convey. However, it continually marks "period of time" as a tautology (which I understand) and attempts to convince me to correct it to "period time" (which I don’t understand).

This sentence is an example of how I’m using the phrase:

"It confirms the fact that such a development can only take place over an extended period of time."

I’ve tried shortening it to "period", but it comes out sounding stupid (for lack of a more eloquent term), and shortening it to "time" makes it sound stilted and strange.

So, my question is this: is "period of time" appropriate to use in a college essay, and if not, what can I use in its place to convey the same meaning?

P.S. I’m new to StackExchange, so I wasn’t sure how to tag this. I put "phrase-usage" as I believe that applies to what I’m asking, but I’m not entirely sure if there was a tag that would better suit the question I’m trying to get an answer for. It would be a big help if someone could let me know what this question would classify as, so I’ll be better prepared the next time I need to ask a question like this.