I've had trouble putting poly on top of epoxy too soon, it's not something to mess with! In my experience, 24 hours at panel temps at or over 70° if only one coat is used, if there are two, best wait a full 48 like Barry says.
Before I realized how important heat was to epoxy, especially when using poly, I put poly on top of 2 coats of epoxy after over 24 hours, panel temps were in the low 60s. The top dried but the underneath was a gooey mess, and stayed that way for days, even after putting the parts in the sun (it was cold, though). I thought i was going to have to strip the parts, but they finally kicked off. It was not fun.