I'm a total noob at this keg thing as well so take this for what it's worth and I'm sure someone else will correct me if I'm wrong.
If by prime you mean naturally carb and condition you would first purge the keg with CO2, then rack over 1/3 less the amount of priming sugar you would use for bottling. Once racked, seat the lid with about 20psi of CO2 and clamp it down.
In this state the keg can sit indefinitely. If not naturally carbing, you want the beer at serving temp, force carb it and again it can sit, I believe.