I would like to stop taking hiv prep.i have taken this for 20 days. if my last uprotected sex happens today, when can i stop this prep thing i am on?

Get re-tested first: A very important consideration before stopping PrEP is to be re-tested for HIV first, and enough time needs to pass after your last unprotected encounter to ensure a valid test result (as little as 2 weeks with modern in-office testing, longer with home test kits). Stopping PrEP before re-testing could result in loss of any protective effect it's having on a recent exposure.