Met a girl tonight, got her number after chatting her up on Facebook. How long should I wait before asking her to lunch? (I'm nineteen if that helps)

My band played tonight and I met her because she came with a fellow band member's girlfriend. I added her on Facebook and we talked a little and I asked for her number. I want to get to know her better, so how long should I wait before I ask her out to lunch? Maybe this is a stupid question, but I think I should you ask you guys (ladies) first before I do something wrong.

Thanks!