I stopped eeping on my tummy at about 25 weeks. It just got uncomfortable and it bothered the baby.
Omce it's uncomfortable for you, your body wont shift onto your tummy anymore, and once you get big enough you just cant lol.
U can sleep on your tummy as long as your body/ baby can take. I wish I could still sleep on my tummy.
trust you will know because it will start being uncomfortable and will be able to feel the baby as you try to lay on your stomach.. It comes naturally even if you not showing yet