The primer can sit/set forever; and, as long as there is no rust under the primer, and as long as the primer surface is not physically damaged (scratched or scarped), that primer surface will last a very long time (and it will stop rusting of the metal under the primer).
However, after the primer is first sprayed on, if you do not spray the next coat within the recommended time-period (usually a few hours to a few weeks, depending on the primer), you then have to scuff the surface of the primer before painting on the next layer/coat of paint.
