Same experience as above - in winter I have also had to wait more than 24h to be able to sand epoxy even though daytime temperatures were about 16C.
I once did a backyard A-B comparison on Bote-Cote versus FGI epoxy (FGI is a large marine fibreglass supplier, generally 5:1 epoxy) in autumn temperatures, using fast hardeners for both. Nothing scientific, but the FGI was sandable well before the Bote-Cote, something like 50% earlier.