I slept on my stomach until it wasn't comfortable which for me was at about 20 weeks. I think it also depends on how big you start getting. I just relied on my comfort level to tell me :)
Yeah I'm a tummy sleeper to! After having my son the first thing I did was tell my husband to wTch and I flopped gently on to the bed tummy first lol was great! So far still can but my belly is growing, soon I'll be on my side again!