Being a man, I cant say for certain, but I would guess a week or two. You should ask your health advisor. But REMEMBER: just because you cant get pregnant doesnt mean there are no risks to unprotected sex. If he has had a lot of unprotected sex there is a chance he may have an STI, or worse, which could be dangerous to you both and potentially stop you getting pregnant in the future. Also, if you sleep around behind his back without use of a condom, there is always a chance you could catch something and give it to him. Not all STI's have symptoms, so him saying "I feel fine" is NOT good enough. Those infections that do have symptoms can always develope later on, when it is too late for treatment and could possibly be fatal. The ONLY time you should have unprotected sex is if you are with a long-term partner who you truly want a future with, and even then you should BOTH be getting checked out at a G.U clinic several times a year just to be sure.