I'm not sure there is a 'correct' amount. Maybe I'm misunderstanding your question. It's your money. Do with it what you will.
Do you mean, like.. you want to occasionally pull out profits and keep building with the rest?
I think this is a personal decision. Personally.. I play with a lot more buyins for my level than the articles here recommend. That way, little downswings have no effect on me, and also if I want to take money out of my poker account for poker related purposes (like buying PT3, or my library of poker books) then I can do that.
I personally would make sure to keep enough in it to allow you to keep playing at the level you want to play at. (I play at NL50, I wouldn't take out so much that I would be uncomfortable with my NL50 roll).
But.. again, it's a pretty personal thing. So long as you have a solid BRM plan, you should be in good shape.