It depends on the primer, but in general, if it is dry enough to sand, and it dusts up, then it is ready for paint. Some primer/sealers can be top-coated in 45 min. too 1 hr.
Also, primers should not be allowed to "heal" before painting. primer healing can occur if the primer is left unpainted for many days or weeks.
Hope this Helps,
rmichael pro painter