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.
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.