I guess the answer to that question is never!
I’ve said it before and I’ll say it again… always, always, get tested with a new sex partner. Forget that they say that they’ve been tested, show their last test results. Get tested together before you have sex. And unless you’re ready to be a parent, getting married– use a freaking condom! People only talk about HIV/AIDS, but forget about the many other diseases that are out there.