Cold casting is a pretty interesting method, though it takes a LOT longer. I recommend putting both halves of the resin in the fridge (or even freezer) to cool them down before casting, even if you weren't planning to set the molds in the fridge, colder resin reacts more slowly, meaning not only do you have less risk of damaging the mold, but you have more working time to stir, degas and pour.
However, keeping molds in the freezer would indeed keep many resins from setting up at all.. If you can put a shaker table in your freezer, this is great, otherwise, meh.
To answer your question, in the fridge, you might increase the set time by 50% to 100%, so don't assume it will just take a few minutes longer.