I've had a broody hen come off of the nest once a day for 15 minutes to eat, drink, and poop. That was un relatively cool weather. In hot weather I've had a broody hen come off of the nest twice a day and stay off more than an hour each time. Both broody hens had great hatches. It doesn't matter what the air temperature is, what matters is the temperature where the embryo is. Those eggs are so dense it takes a while for the embryo to cool down. If you are candling those eggs in a gale force wind in temperatures well below freezing you might run into a problem but under normal circumstances you will do no harm.
When I candle mine (usually 28 eggs) I turn off the incubator, carefully remove the eggs, remove the automatic turner, clip the screen in the bottom of the incubator so they have good traction when they hatch, refill the water reservoir, and then I start candling eggs. After putting the good eggs back I turn the incubator back on. I haven't timed this but it takes much more than a few seconds.
To me the biggest danger in candling is getting in such a hurry that you drop an egg and crack it. Take your time to do it right but don't mess around. Just safely get it done.
