I followed up with another vendor, System Three, and they said at the bare minimum, the epoxy has to be cured enough to be sanded with creating any gooey little balls. So if you sand it and it makes dust, you should be ok.
Waiting longer (up to 5-7 days) can be beneficial to ensure that cure stage but be aware of amine blush issues.
The time between when I put the last coat of epoxy on the deck and the first coat of varnish was about five days. The epoxy I am using (Bote Cote) is a two to one mix and the manufacturer claims that it will not get amine blush. The reports I have read in local forums and boat building magazines, have been very favourable of this product, but I guess time will tell.
I was very tempted to launch the boat and give it a trial prior to varnishing it but I thought it would be nice to have it finished before I use it.