Of course they work, why wouldn't they? As far as the girl knows, you really do have to go meet up with your friends, or whatever other bullshyt excuse you gave her.
There isn't an approximate number of minutes. That's the beauty of false time constraints. You only have to "rush off to work" when you're ready to go. The number of minutes is however long you take to get the phone number and get out.
The conversation doesn't have to be thrilling, as long as its not boring as all hell. Just use fluff. "Hi, what's your name, what's your major, where are you from, etc etc." That's all you really need. Then when you're ready, pull out your excuse for leaving and grab her number on your way out.