I see her once a week and I've only talked to her a couple times. I'm not 100% sure she's interested or if she's just being friendly. I don't want to ask too soon, but I don't want to ask too late either. Remember, I'm talking about a girl who I see during the day at school once a week for a few hours, not some nightclub thing where I'll only see her once and never again.
Ask her out next time you see her. There's no such thing as too soon, she either already has some interest in you or she doesn't. That's not going to change by waiting. If she's going to say no, I can bet she would say no regardless of whether you asked now or later. Same doesn't go for the other way around though...if she has interest and thinks you only like her as a friend, she might pursue/be pursued by someone else and then it'll be too late, so might as well go for it!
Don't wait...but keep it informal...ice cream cone or coffee or something light and easy...if that goes well then maybe try lunch or something...would be different if you knew her...but I used to try to take some stranger to a more formal setting...it is really intimidating..o.O