First off, follow the manufacturer's recommendations, if any. Otherwise, as a general rule of thumb, wait at least until the paint is tack-free.
Some coatings, such as Future and some glosses, need different standing times. Some glosses need to be recoated at a specific time, or after full cure. Future can be applied on top of Future as soon as it's tacky, but should be cured 48 hours before overcoating or masking with other paints.