If you met a girl in class (college), and you've talked a little, and always smile at her, but she's shy -- how long do you typically wait to ask for her number? do you wait for her to seem more interested?
I usually wait for a "sign of more interest". If I'm talking regularly with a woman, I would ask her for her number after about a week. I tend to take my time if she seems shy and slow to come around. Someone who flirts soon after meeting me, gets asked for her number much sooner.
I know this is a guys question, but, this guy I liked asked for my number and I was shy, but it made it easier that he made the first move, and it was quite sweet :) then I got to know him better. make sure she looks like she likes talking to you if you know what I mean, then just ask, after like a week is good I reckon if you talk all the time :)