It depends on your skill level (which is correlated with winrate, obviously) and your "comfort factor". If you can play solidly while under-rolled (i.e. you don't become "scared money" when half your roll is in play, and/or you can deposit new funds if you go busto) then you can take shots whenever you feel like it.
I've built rolls up from 5 buy-ins. I've also had downswings of 15-20 buyins. (And I've only won about 15 buyins in my last 100,000 hands; a "breakeven" stretch which has destroyed my passion for the game).
This graph is from
my PG&C thread where I used an aggro/crazy BRM strategy of "Got 5 buyins? Let's move up!"
On a 14bb/100 heater, I went from 4NL to 50NL in <25,000 hands. With "standard" BRM, it would have taken me about a quarter of a million hands.
FWIW, I've since moved back down to lower stakes, because I found the swings were too mentally taxing at 50NL. It was quite fun taking shots though.
In my experience, if you don't take aggressive shots, you get "trapped" in the high rake environment of the micros for years, and it eventually kills your desire to play.