All babies develop at slightly different speeds depending on a lot of developmetal factors however a round about time in which babies will begin to roll over can be estimated. Some babies can begin to roll over as early as 4 months. This usually happens when they are able to kick themselves over from their stomach to their back.
Additionally most babies will be able to flip from their backs to their stomachs around 5 to 6 months old. It takes a bit longer to flip from the back to the stomach as it takes stronger neck and arm muscles in order to perform this maneuver.