I agree with others, you will know when. But you still may wake up that way. You're baby is protected.
I still wake up and I am laying on my tummy. 😂 I try to not even though it's not uncomfortable yet I just feel like I am squishing baby.
When baby tells u no more i slept on my belly till i was about 18 weeks :)
You will know when you can't anymore... it becomes really uncomfortable!
I slept that way until 25 weeks then baby said no more