The short answer to your question is: Whenever she can get onto her belly by herself, which is usually after six months of age. Babies are put to sleep on their backs or sides because it has been shown that belly sleeping increases the risk of SIDS (Sudden Infant Death Syndrome, commonly called crib death). Since doctors have been instructing parents to put their infants to sleep on their backs, the rate of SIDS has dropped dramatically.

SIDS is rare before three weeks of age, reaches a peak at three to four months of age, and then diminishes markedly after six months of age (though there is still a very, very small risk after this time). The cause of SIDS is not understood but is felt to be related to some sort of interference with breathing when face down.

From a developmental perspective, babies generally don't have the motor control to change their position much early on. Most babies start to roll over at about four to five months. They first learn to roll from belly to back, and then several weeks later from back to belly. Once a baby gets very good at doing this at will, then if he really prefers to sleep on his belly he will flip himself over.

You should still put a baby to sleep on her back at this point, as there is still a very small SIDS risk after six months. However, if she really objects to it and continually flips herself onto her belly, I would not recommend trying to turn her back.