It completely depends on the person. I personally play cash with 35-70 buy ins, i'd play mtts with about 100-200 buy ins and i'd play sngs w/ 50-100 buy ins (i don't actually play sngs/mtts anymore tho).
But it depends alot on the stakes you play. I play nl100 cash and for me i need to be well rolled just to be able to stand the swings. I played nl25 w/ slightly less than 20 buy ins, because i knew it was incredibly easy to beat and because the money didn't mean as much to me. I started nl50 when i hit exactly 20 buys ins, but i waited alot more before i moved to nl100. I will wait even more before i move to nl200 (at least 50 buy ins.) When i was playing micro sngs, i also played w/ something like 20-30 buy ins, because again they are easy to beat and the swings are easier to take because it is less money.
So you have to decide for yourself. Basically the higher the variance, the less % of your roll you should be playing with at any given time. Mtts then sngs then cash is the order from higher to lower variance.