How long do you wait until you ask a girl you met through a friend? I mean you'll prob. see her again?

I don't jump in a pool with a bunch of sharks...I feel I need to ';investigate'; but I do know i'm attracted to herHow long do you wait until you ask a girl you met through a friend? I mean you'll prob. see her again?
depends if yu see her evryday i'd say a month n a half