Hens often get off the eggs for an hour or more, and then still hatch them just fine. I've had hens repeatedly get back on the wrong nest, and eggs get chilled over and over, and still hatch. On time, even. I've had eggs get chilled, get egg goo all over from broken eggs in the nest, wash them, put them back, have that happen repeatedly to one clutch, and by the time hatch day arrived, there were only 6 left, out of 12. I thought for sure they were all dead, but 5 of the 6 hatched.
Relax. Don't worry so much. If the hen gets off longer than you think she should, just stick her back on the eggs and let it go. You may get a good hatch anyway.
Some hens, if they've been broody way too long, will leave the eggs before they hatch. I recently had 2 broodies that I was saving for some turkey eggs, but with various problems that arose, they didn't get the eggs until they'd already been broody nearly 3 weeks. One of them abandoned the eggs after 3 days on them. I put what I could in the 'bator (which was nearly full at the time) and the rest under the other hen. Turkeys take 4 weeks instead of 3. Only 4 turkey eggs turned out to be viable, so I put those 4 and 9 guinea eggs under the hen who was still brooding. The night before they were due to hatch, I heard peeping under her, and thought great, I'll have a nest full of babies tomorrow!
Normally, a hen will stay on the nest at least 24 hrs. after the eggs start to hatch. She didn't. The next morning, she was off the nest with the 1st 2 hatchlings, the rest of the eggs were cold. I put them in the incubator, but it was too late. No more hatched. So she's got 1 poult and 1 keet, and I lost the other 3 poults and 8 keets.
I'm sure it was because she'd already been brooding for 7 weeks by the time they started to hatch. Her hormones were telling her it was time to quit. So don't count on a hen staying broody as long as you want, it doesn't always work that way. They do have something of an internal calendar to tell them when to give up, otherwise, they'd stay on dead eggs until they starved to death. Hens get off the nest daily to eat, drink, and poop, but they don't eat nearly enough to keep going indefinitely. Brooding's hard on a hen, they lose weight and condition. They handle it fine for up to about 5 weeks, but I wouldn't push past that, if I could help it.