My "bankroll" I consider essentially to be the total amount I am willing to lose if necessary. Although Poker involves great skill, at it's core it still is gambling. It's entirely possible to have such a terrible run of luck that you wipe out your entire bankroll, no matter your poker skill. These things occasionally happen.
This being said, my bankroll is around about $2k. I never join tournaments with a buy-in more than $100. I never join straight-up tables with more than $200. Ever. Golden rule.
In my 10 years or so of playing poker, I'm in total profit of around $40k. I withdraw my winnings anytime I have more than $1,000 in my balance. My bankroll from about year 3 has always been $2k, and I've never altered it. What I mean by this, is if I were to ever reach the point where my running loss as such was $2k, I would pack poker in and thank the poker gods for the money I've made over the years.
By sticking to this strict bankroll system, poker has always been a hobby for me, never cost me money I couldn't afford, and always been a generally enjoyable experience. I hope it lasts for many years to come.