This is a debatable topic, and there aren't many wrong answers.

However, there's two things we're worried about here: confidence and variance. 10-15 buy-ins is probably enough to deal with the variance at very low stakes (<25NL) because the players are so bad. However, you may want 20 buy-ins to be comfortable at 25NL+.

20 buy-ins is good for low-med stakes inmho, but I'm a bit conservative. I moved up to 50NL at $1000 but waited until $3000 for 100NL. Part of this was a lack of confidence in my game. 30 buy-ins gives me the confidence I need to play good poker.

I am a big fan of moving up as soon as you reach the bankroll requirements. This allows you to maximize your profit and learn the game at a new level. I stagnated forever at 25NL before moving up. But each level since then I have learned much more quickly.

As an aside, bonuses make it possible to move up more quickly than your skill level dictates so I could see playing minumum 10k hands per level before moving up.