I'm not currently withdrawing, as I'm re-building my bankroll (I stopped playing a little over a year ago, and just got back in the game in July). When/if I reach the point I used to be, I'll be withdrawing about $100-$200 per month. But dman is right, how much you withdraw should depend on what your goal is; if you're happy with the limits you're at, then go ahead and cash out your winnings (many recommend keeping a bankroll that puts you at about 300-500 big bets of your limit).
If you want to reach higher limits, then let it sit and build itself up.
If you're somewhat inexperienced (and not yet sure where your comfort-limit lies), then I'd recommend this:
Let's say you start playing at $.25/$.50 tables. You should aim to have about $150 in your bankroll, and work yourself up to at least $300 before moving up to $.50/$1. If this is the first time you're moving up, I can recommend cashing out 50% of your earnings, to accomplish two things:
1. You get more experience before moving up. Experience is completely invaluable, and the more hands you play, the better. I think most will agree with me on that one.
2. You get to reap some rewards for your work. A suggestion for what to do with the (relatively small) cash out is to buy yourself a poker book.
So for every $100 you increase your bankroll with, withdraw $50. It'll take you a while longer to reach the next limit, but if you're a winning player you'll still get there - and if you're not a winning player, perhaps it's not time for you to move up anyway. This way, if you do move up in limits and take some beating and realize that you need to move back down, you can still smile about the cash outs you've made.
Also, it's a nice feeling to have something to show for the hours you put in other than a number on the screen. If you're married or live with someone, it also tends to make them more understanding if you can show that there's a gain other than your personal enjoyment of the game. Your mileage may vary on that one, though...