In hot weather I've seen a hen come off the eggs for over an hour at a time twice a day. In cooler weather I've seen them only come off once a day for a max of 15 minutes. They all got good hatches. Sort of like HE said, it's not just the weather, it depends on the individual hen too. Some I never see come off the nest, but they don't poop in the nest so they have to be coming off to eat, drink, and poop. Since a broody holds her poop until she comes off, you will often fine a huge smelly poop, really huge.
Some hens are easier to confuse than others. I've had many that always went back to the right nest. I've had some that were more challenged in that regard. I want to say that it seems earlier in the incubation is more of a trouble spot, but I had a hen that had been on the eggs two weeks with no problems go back to the wrong nest. By the time I saw what happened, those eggs were ice cold. I just put her back on the eggs. She hatched 11 out of 11 and was a full day early. You never can tell what will happen.
I built a couple of my nests so I could lock a hen in there if I wanted too, plus there is a side area that can hold food and water plus room to poop I can open up. I've used that to move a hen from a nest I didn't want her to brood in, just locked her in there for a few days with eggs, then opened up. That's always worked for me, but I've only done it a few times. The first day she is in that new area, I leave her locked in that dark nest, not opening up the side area at all. I think that dark helps them accept the move.
One time I had a hen that started out confused before I gave her eggs. She was just hopping from nest to nest. I locked her in a nest for a few days and gave her eggs, probably waiting a week before I opened the door to let her roam with the flock. That was a day I could be around and watch her. After that she always went back to the right nest.
Most of my broodies never have this problem, but occasionally some do. Usually it gets better further into the incubation but not always. None of us can give you any guarantees with this stuff. With mine it has always worked out, but sometimes I've had to help it along. Usually the broody does all the work, that's nice. But sometimes they can be frustrating.
Good luck!