I withdraw 50% of the monthly profit, but a minimum of 100$, even if i have a losing month.
The minimum 100$ is important to keep you motivated for being a poker player, and of course can vary depending on what stakes you play and what you need the money for. It has to be enough so you can do something with it, but not enough to make you feel satisfied.
I've been doing this for a year and i'm relatively happy with this method, but my bankroll is at the same level as it was then, i guess i'm not your definition of a "grinding machine".
You should start withdrawing money regulary when you also play a lot and you're not in the microstakes-there you desperately need to climb up stakes.