Relax. They should be fine.
How long the hen can stay off depends on a few different things. How cool is your weather. Probably pretty warm right now. In cold weather I’ve seen a broody eat, drink, and take her daily constitutional in about 15 minutes. In the heat of summer I’ve seen a hen stay off the nest over an hour a couple of times in one day. So weather plays a part.
How well are the eggs insulated? In most nesting material, probably pretty well.
How close to hatch date are they? The more developed they are, the more heat they generate on their own. In those large commercial incubators, the ones where they may have 60,000 eggs in one incubator, the problems they have when it gets close to hatch is cooling the incubator, not heating it up, due to the heat those eggs put out.
I’ve had a hen go to the wrong nest. When I found her, the eggs were ice cold to the touch. I just put her back on her eggs. Not only did she hatch 11 out of 11, they were almost two days early.
I really don’t think you have much to worry about. Good luck with the hatch.