HUD has a website where you can look up fair market rentals for a given location. Here's a link:
https://www.huduser.gov/portal/datasets/fmr.html#:~:text=Fair%20Market%20Rents%20(FMRs)%20are,the%20...
As @Mike9241 said, the IRS does not have a clearcut rule on this, but obviously the further your rent is below FMV, the more likely it is that your return gets questioned.
There are court cases that accepted 80%, and there are court cases that didn't.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
