Gross motor skills, . using large muscles of the trunk & legs, progress head-to-toe the 1st year. A baby who gets her head & chest up when on her tummy ( in prone position) at 4 mos. starts rolling tummy-to-side. At 5 mos. she props on her forearms & starts to roll front-to-back. At 6 mos.she rolls both ways. You can't set her in her car seat on a counter safely from 4 mos. on. "Tummy Time" is safest on the floor.
4 months. Most babies who were born at term will start to roll one way or another around 4 months of age. Some are not consistent or may not make it all the way at 4 months of age, but by 6 months of age, most babies will do this on their own. Premature infants typically will take longer than that depending on their level of prematurity.
4-6 months. The "book" answer is: front to back at 6 months, and back to front around 7-8 months. Most children that i've seen start rolling back to front (since that is how they are most of the time, on their backs) around 5 months, and front to back at some point later when they get sick of being there. Sometimes the larger children and the more "easy going" may roll a little later than others.