So, what about a comfort zone? An amount of money that allows you to go on a terrible streak of luck, losing for week, if not months. Well, let's look at the stake level you play currently. For example, say it is $2/$4 Limit Hold'em. The general consensus is your bankroll should be 300 times the big blind. So in this instance, you should have $1,200 to stay comfortable.
Limit Hold'em is one variation. But, what about the much more popular No Limit Hold'em? Here you'll need much more money than the equivalent Limit variation. In $2/$4 No Limit Hold'em, often the maximum amount you can sit down with will be $400. Whilst in $2/$4 fixed limit you may sit down with $100. An accurate bankroll figure would be impossible to calculate as different variance applies for different players, a loose-aggressive player may need more of a comfort zone than a rock, for example.
Needless to say, the bigger your bankroll the better. Many part time or hobby players won't need any bankroll at all. If you have a regular income and set aside a percentage of this each month for playing poker, you'll never go broke assuming the amount set aside is reasonable enough for you to live a comfortable life if you lose each month.
If you take your poker seriously, you should manage your bankroll carefully. Don't play in games that are too big for you and don't be afraid to step down the stakes if you're in the midst of a bad run.