The unwritten rule that a guy (or gal, for that matter) must wait a minimum of 2 days before calling the woman who's number he managed to get. An earlier call shows desperation, the longer one waits the better, however pushing it too long tends to make the woman upset and believe you are cocky. It's a delicate balance.

More rules and the ways in which they can and cannot be broken may be found in the movie Swingers.