the idea is to equalise the air pressure within the fork at rest with the outside air. so theres no real length of time.
MOST people have the bikes weight off the fork. if you dont, there will be less air inside as an airspring... in theory, a possible tuning aid if you keep track of it, but i dont know of anyone who does it that way.
i have been told that if you bleed it with the forks compressed, a slight vacuum forms inside when you extend the forks.. and supposedly because the seals are not designed with that direction in mind, you can have a premature seal failure.... but i dont know if thats bogus info or not. its just shit ive been told online. while we're on the subject, can anyone clarify that for sure?