how long should i let a female marijuana plant vegetate before i can snip a piece off to clone?

Well how do you know it is a female if it is still in veg?

I've heard that clones do better if the mother was flowering but if you can already tell it's a female and it has preflowers and branching then I think it's probably fine to clone.

when the branches are about 6-8 inches long, you can clone, make sure you have your rooting gel and humidity dome ready before hand, the longer you let the clones sit out the less chances they have of living, also if your using HID lighting do not put them directly under it, but maybe 4-6 ft away.