There are two clear answers to this:
- She must not only be called immediately, but she needs to be the one who puts her number in your phone, so you can verify the number works. If there isn't enough interest for this dorky parting game, there's not enough interest, period.
- For the second call (okay, technically the first real call), the next day, and as others have well covered, with a generous attempt to accomodate both your schedules… Again, figuring out when she's free to take a call the next day gives you a few more things to squeeze from that initial conversation… Be subtle, but informed enough to know when to call.
How long are you supposed to wait to call a girl after she gives you her phone number?