When can you consider yourself winning at a certain stake?

Like topic, at what sample size can you be certain of that you're a winning player at NLX?

I've got 340k hands at 2nl at 7bb/100 and 340k at 5nl at 2.5 bb/100 so there I know for sure I'm a winner (though a small winner).

But when it comes to 10nl I've only got around 35k hands and 10bb/100 (30bb/100 last 8k) , so I can't really be sure I'm beating it yet - can I?

The reason why I'm posting this thread is because of the whole move up thing. My bankroll is enough to play 25nl, but I want to make sure that I can win at 10nl before even trying to move up.

So what's a good sample? Around 100k would probably be enough?

Honestly 10k is usually enough, barring WR-distortive swings. At 10BB/100 you probably have enough of a sample already that running above EV isn't a big issue.