There aren't any "rules" as such to any of this stuff, aside from the basic pleasantries such as being authentic and honest, and the default setting of "have fun". As such getting a phone number (or giving her yours) can be done any which way you like.

Personally, I prefer to get hers. It keeps the ball in my court - I'm in charge of persuing the interaction through the telephone rather than leaving me wondering whether she will call (and ultimately getting bored if she doesn't). Getting a girl's number is remarkably easy. Provided you have a good interaction with the lass, just hand her your phone and tell her to whack her number in, with the premise that you'll call her so you can hang out. Simple. I then ring her phone for two reasons: firstly, she'll have my number (and hence know who is calling), and secondly, I can be sure she's given me the correct number and not a false one (this has happened. I was not amused).

That's my way (and the way of many others), by all means copy it or use it as a template, but try and take away the fundemental idea that whatever you think is an acceptable way to do it is the best way to do it. Believe this and she'll believe it too.

As for how long to leave it before you call, call her whenever you feel like. Don't feel pressured into doing it or try to act aloof by leaving it for 3 days, playing games like this is daft and makes you look like a petulant child. Rather, you're an alpha male. You see what you want and you take it. To this end, call her when you feel like it.