Figuring out when your are ready to move up to the next buy-in is one of many challenges poker players face. The advice on this area can vary a lot, but here is how I think it should be handled.
First of all, you have to believe you would be more profitable at the next buy-in. Some players move up whenever they think they are profitable at their current buy-in, but that strategy will lead to being break-even at best. To move up, your winrate, whether it be ROI (Return On Investment) in tournaments or BB/100 (“Big Bets” per hundred hands) in cash-games, has to be big enough that the inevitable decrease in winrate due to tougher competition on the higher buy-in still results in a higher hourly rate (in terms of dollars). It is, for example, profitable to move up if the buy-in is twice as high, and the ROI or BB/100 is expected to be reduced by less than 50%.
How much of the ROI and BB/100 do you lose from moving up a level? It depends on a lot of things, but I would say the ROI drops on average 2-3 percentage-points in Single-Table Tournaments per buy-in. You would have to ask other people for figures for cash-games and Multi-Table Tournaments, as I don’t have much experience with those.
The problem is knowing how profitable you are at your current buy-in, though, because without knowing that you can’t know how you would do on the next buy-in. Some say you need to have played thousands of STTs or MTTs and tens of thousands of hands in cash-games for the luck to have evened out, and the results to be representative of your true edge in these games. And they are right, but if you were to play that many hours per buy-in you would never get anywhere. You just have to make a qualified guess as to what your winrate is, and this can be hard to do since many players tend to overestimate how good they are.
Then there is the matter of how big of a bankroll (the amount of money you have set aside to play poker with divided by the size of the buy-in) you need to play at a certain buy-in. You can be the best player in the world, but if you move up so many levels that you are playing STTs with a 5 buy-in bankroll, there is a big chance you will lose it all from a little bad luck.
A rule of thumb is that you need 30 buy-ins to play STTs, 100 buy-ins to play MTTs and 20 buy-ins to play NL cash-games. These figures are considered big enough to ride out bad luck spells, so you can keep on playing on that buy-in because the luck will at some time change. A very scary scenario using this bankroll strategy is in cases where you have overestimated your skills, and you are in fact a losing player at this buy-in. If that is the case, you will slowly but steadily lose your entire buy-in.
I think it is wiser to have less strict bankroll-requirements and a more flexible approach. Say buy-ins of 20, 50 and 12, and instead drop down if it sinks below say 12, 30 and 8. This lets you move up in buy-ins faster early in your poker-career where it often is the bankroll and not the skills which limits how high buy-in you can play.