50k hands would be an ok sample to get an idea of a relevant winrate, 100k would be even better but as low as 20k would be some idea keeping in mind could have been a heater involved for it that skews how good you actually would be over a significant sample at the limit.
As you move up limits essentially you will come accross better players and therefore in general the bb/100 winrate is often going to decrease a little. However that being said probably no reason you couldn't have similar winrates especially at 2nl, 5nl, 10nl.......I have seen in a thread yesterday that on stars if you can maintaine a 4bb/100 winrate over a large sample like 100k+ you are probably crushing it (i could be remember that wrong, maybe its 4BB which is 8bb, anyhow you get the idea higher the stake lower expected winrate but better hourly rate (ie profit) in essence if winning.
Moving up limits has more to do with skils and br than it does wheather you have put in a proven winrate at a level, especially when talking the lower levels like 2, 5, 10, 16, 25. Only because it is very possible to have built your br at any given level to move up prior to ever achieving a relevant sample of hands (especially at 2, 5nl).
Hope this helps, I'm sure there are some cash grinders who could input if I have steered you wrong here.