[tsubaki]

Usually, if you can put your hand on it and not have it stick to you it will work.
Normally either using a scuff pad or water sanding (or both) will let you sand between the suggested drying time.
If when sanding, if it sticks, becomes gummy, it's not cured enough to sand.
One thing about paints, primers, resins and epoxies is if it has a glaze or skim coat on it, it takes a little longer to cure out. Scratching it or sanding it increases the cure rate (makes it quicker).
The Zinc Chromate I've used usually is good after 24hrs to be sanded, 45 minutes or so for repaint. Keep in mind, we have high temps and humidity, which increases drying rate.