When did your LO star sleeping on their tummy? How did you know they were ready?

Our LO just learned to roll onto her tummy but has yet to learn how to roll back. For the last 3 nights she has been continuously rolling over, getting scared an starting to cry hysterically. We/ I have been flipping her nearly every hour all night. At 4am she flipped but didn't cry and just fell back asleep. I waited 30 minutes until she was fully asleep and flipped her. They say back is best but until when?