If you're a pro or want to become one, get in the habit of having 500 big bets or more ready for play. 6 months living expenses saved in the bank just to be safe. But if you aren't trying to be the next Phil Ivey, and you can reload or are willing to move down if you hit a rough spot, 300 or even 200 big bets is fine. If you are a losing player, no amount of big bets will save you, so make sure you are always working on your game too.
One often overlooked aspect of bankroll is rakeback and bonuses. Make sure you take full advantage.