I met this girl 2 weeks ago. We had plans to gout on Thursday and she told me last minute that she had to do something for her sorority and that she would like to meet up later that night. Anyway we met up briefly that night and she told me to call her to go out sometime.

I called her on Saturday (left no message) and I didn't get a call back. I would like to meet up with her to get to know her better but I don't want to come off as needy. Is calling back the next day fine? Or should I wait longer?

I would give her another call in a day or so, and leave a message this time. Just say you were wondering if she would like to get together sometime, and leave her your number. After that, don't call her anymore. If she is interested, she will call you back. If not, then you know she isn't.
Good luck!

Next time leave a message so that she knows to call you back. That way, you don't have to play the guessing game.

Don't call her more than once or twice a day. I say call her the next day and leave a message. If she doesn't respond in the next day or two then she isn't interested. But if she calls back or responds to you in any way she is interested and you will have a chance. Just remember it is what it is and some things happen for the best... good luck (:

Call her back only one more time, as you don't want to come across as a pest. This time leave her a message and a phone number.

If she doesn't return your call, it's a safe bet that she isn't interested.