Dd2 only ever rolled one way, and was rolling by 6 weeks, but that was because she is missing a hand on one side, so that was much easier.
Dd1 didn't roll until 10 months and rolled both ways.
Ds rolled only one way, was rolling about 5/6 months.
I think it's fairly typical.
One of my friends had a funny story about that. Her dd as a baby only rolled one way, but in such a way that she rolled along rather than rolling back to front, then front to back and ending up in the same place. But she'd never used this as a way of progressing, didn't really roll much anyway until she went away one time.
She went to stay at a hotel when she was about 7 months, and left her playing happily in the middle of the floor while she had a quick shower. Came back into the rool in about 5 minutes, and no baby to be seen. Searched the room, called reception who sealed the hotel, police called. Standing distraught in her room with various members of staff calming her down when she heard a little baby noise... turned out baby had rolled, and kept on rolling all the way under the bed until she reached the wall at the far side, where she was stuck!
Lots of 
and very relieved mum, hotel staff and police.