I can still lay on my back & feel comfortable & I'm 33 weeks. It's just whenever it's uncomfortable for you.
it all depends on whats comfortable for you. same with sleeping on your belly. If it doesn't make u uncomfortable it doesn't affect baby. I was around 20 weeks when I had to stop belly sleeping. It wasn't until about 34-35 weeks that it became uncomfortable to sleep on my back. (I am currently 38+4)
When the baby gets bigger and you feel him/her move a lot