At $25nl, I am 11BB/100 over 27k hands and at $50nl 7.5BB/100 13k hands. Both are fairly respectable and would consider them better than average. I do, however, have a lot of room to improve so they could be even better than that. I'd say that 25k hands is the minimum you want to play before making any long-term declarations about how profitable you can play. I've played mostly Poker Room and Stars and did not see much of a difference in winrate although the players are very different.