Six months. When babies used to sleep on their tummies, we expected them to roll over by four months. Now that we know it's safer to sleep on their backs, they do roll over later-- by six months.
4 months. Many infants are trying to roll over by 4 months. Encouraging supervised tummy time is a good way to build your infants upper body strength and to help foster the skills needed to roll over.
4-6 months. The textbook says babies roll front to back by 6 months, and back to front by 8 months. However, many babies are rolling over starting at 4 months in either direction. Those that are a little more sedate, or are more contented in one position may roll later rather than sooner.
4-6 months average. Rolling should start by 4 months and be consistent and better by 6 months. Those are averages and there is a range of 'normal', so discuss with your pediatrician if there are problems. Lots of tummy time will always help accelerate things.