The Cotton Swab Test
When is the best time to apply the next coat of resin? It all depends. But if your schedule allows, you will get the strongest bond when the prior coat is not yet cured. But if you do it too early, when the prior coat is still wet, you may float up the fiberglass, or you may not create the build you want. If you wait too long, you will need to sand.
MAS Epoxies suggests a "cotton ball" test. When the epoxy is still wet the cotton ball will not stick. When the resin is too dry, the cotton won't stick. In between this two extremes is a period when the epoxy has stiffened up somewhat but is still tacky. During this period, the resin will grab the cotton fibers and pull on the ball or swab. This is the best time to apply another coat.
While the epoxy is tacky, new coats will not mix in with the prior coat, or get under the fiberglass. If you wait longer it is not a big deal, but you will need to check for blush, and clean it off, and then you will need to sand. As long as you can push your thumb nail (the "rule of thumb") into the dry resin, you do not need a heavy sanding, but a light sanding will promote a better bond.