2k epoxy primer usually has a long curing time before it’s fully polymerised. This is the ideal time for adding others coatings without the needs for scuffing to aid adhesion - subsequent coats are chemically bonded to the epoxy. Download the tds for whatever brand of ep you’re using, though I’d say you’re grand
I was told a general rule for chemistry, is that for each 10c increase, halfs the time for a reaction, which seems to line up with paint TDS timings.
My attempts I found it took much longer than stated and thats with a heat lamp trying to speed things up and putting in a heated room. I needed it to fully harden to sand back my cockups...
I think you need to refer to the tech sheet.
Most epoxy primers have a strict temperature range for curing and if you go below that it may never cure. Sometimes it will continue curing once it warms up again, it might not though. I had a nightmare trying to cure HB989 epoxy primer in warm autumn weather (8c at night, 15C in the day) even with panel heaters applied for the first few hours.
In winter it's always best to use a fast or winter grade hardener and use panel heaters for atleast a few hours after application to ensure it does cure, it also means you can wet on wet it with other products more quickly.
Another issue with winter use of epoxies is amine blush which is a waxy residue which forms on the painted surfaced when epoxy hardener amines react with moisture in the air- common issue in cold and/ or damp weather when using epoxies so another reason to use a fast hardener and get some heat on it as soon as its flashed off. The only epoxies I would trust to not amine blush are ones designed to be used in cold and damp environments like jotamastic wintergrade.