Many babies roll from front to back around 2-3 months and from back to front around 5-6 months ... but some skip rolling alltogether. We thought that Brooke was going to skip rolling but she has has a new interest in it the past few weeks. Two weekends ago (the first weekend in May) she rolled over both ways in the morning. She would roll once a day or so after that.
Then we moved to her trying to get onto her tummy at night. After one night of sleeping on her tummy for a bit she hasnt done it again. Perhaps she thought she was a tummy sleeper but found out she wasnt ;)
Her new thing is rolling onto her tummy from her back every time she gets bored, and then crying/complaining that she is on her tummy. She is less comfortable with her ability to go from tummy to back and occasionally thinks she is stuck when on her tummy (though she can easily roll to her back, and in fact, getting to her back is easier then getting to her tummy).
So at 7 1/2 months I can say we have almost mastered rolling. She is a bit unsure of herself, but knows that she can do it. Back to work on some more skills :)