- Is it based on a certain percentage of winning sessions over a certain period of time?
- Is it based on a certain amount of profit? Number of buy-ins won? BB or bb per 100 hands? per hour? per day? per month?
# winning/losing sessions is completely irrelevant. Your BB/100 is equivalent to your $$/hour when you adjust for your hands/hr, $$/day (hours/day*$$/hour), and month and buy-ins won (50BB = buy in). And so it is just used to gauge how well you are "beating" a level.
As for number of hands, I'd say you need probably 30k, unless you are crushing the level and not running extremely hot. It then depends on factors such as number of tables you are playing (more tables = less win rate/100 normally but your $$/hr might be more since your hands/hr is more).
If you are able to say run at 3BB/100 at 500 hands/hr, you are probably beating a level fairly well. It depends on stakes, however, because to beat 0.10/0.25 or 0.25/50 for 3BB/100 means that there is a large room for improvement, because these levels are chock full of very poor players. Whereas if you were beating 2/4 for 3BB/100 you are doing extremely well indeed.
But ultimately it doesn't really matter if you are "beating" a level or not. Try to find a level where you can handle the swings and have a decent win rate.
Ultimately, the ONLY thing that matters at the end of the day is $$/hr (ie. BB/hr). So if you get 1000 hands/hr in at 2BB/100, you are doing a better job than if you were winning at 10BB/100 while only playing 150 hands/hr.