There's no set time, really. Make sure that she takes your number too, that way she's more likely to answer.
I normally send a text shortly after getting the number, My favourite is one I got off the forums I think "Hey, you have my number now, but you're only allowed to call me ONCE a day, k?

"
Girls are a lot less likely to give their numbers out if they've only known you for 5 mins, and even less likely to answer when you do call, so set up an instant date: "Listen, I have to get to class but how about we continue this over sundowners at 5 at place x?" If she agrees "Cool, let me just get your number in case I get held up"
If you can't get the instant date or if you forget, text her a bit or call her but keep the conversation light hearted and fun. I hardly ever call, to be honest, but that's the social norm here... Always be the one to end the conversation, and ALWAYS end it at a high point. This shows that you've got other things to do and it leaves them wanting more.
As for facebook, I'd wait for her to add me rather.