You can sleep on your stomach for as long as you want, just make sure you're not uncomfortable or in pain, because if you are, baby is. I'm 29 weeks and sleep on my belly still, not completely on it. Some nights I can't sleep on it at all because of where he is positioned so I just sleep on my side.
They say you shouldn't sleep on your back after 20 weeks. I stopped laying on my stomach when I just barely started getting a little bump cuz I wanted baby to have as much room to grow as possible and didn't want to squish him. I never really was a stomach sleeper before but I'm 39 weeks now and have been really wishing I could but I wouldn't dare at this point lol
You don't really have to, but whenever it gets uncomfortable. I still sleep on mine from time to time and I'm 39 weeks.