Depends on the epoxy, but most recommend waiting at least a month before coating the floor. A month is usually how long it takes for the concrete to "fully" cure.
I let mine cure for four months before having Home Pro Floors (www.homeprofloors.com) show up and do their thing; it was a new home and we hadn't moved in yet, so I didn't have to haul stuff out of the garage so they could work.
