I like moving up when you have the roll to do so AND you have beaten the previous level for a decent amount over a decent sample.
In both cases the exact numbers will depend on the individual.
An example might be that once you hit 5NL, you move to 10NL when your roll is at least $250 AND you have played at least 20K hands at 5NL with a winrate of at least 4ptBB/100.
Tailor numbers to suit.