A lot depends on how thick you apply the primer and the temperature and humidity to determine drying time. Under normal circumstances where the whether is dry and is 70 degrees or more and you put a thin coat on, it should be dry enough to recoat in an hour. If the weather is damp and the temperature is above freezing but cold it could take a week to dry especially if you apply a liberal coat. The best way to tell if it is ready is it will need to be scuff sanded before topcoating. If the primer gums up on the sandpaper, it isn't ready.