Belly to back, 2ish months. Back to belly, still not there. I try not to compare, but my first rolled back to belly the day he turned 4 months. Carter rolls so easily to his side and almost gets there, but then goes back to his back. Its very frustrating to watch.
Still waiting, and DS is 5 months old. My DD did not roll either way until 7.5 months, though, so I am not worried. Both of them can/could sit unassisted with their hands not on the ground before 5 months. Every baby is different and rolling is a really random milestone that depends more on temperment/desire than ability a lot of times.
Re: Rolling over...
Tummy to back when he was 2 or 3 days shy of 3 months. Back to tummy at 4 months.