When they hatch the chicks absorb the yolk. If they are healthy they can live at least 72 hours off of that yolk with no more food or water. At least 72 hours, not a bare minimum 72 hours. That's why chicks can be mailed to us from the hatcheries.
When the chicks get hungry or thirsty they will tell the hen with their chirping. The hen and chicks talk to each other a lot from when they internal pip forward, before they even external pip. I don't speak that language so I don't know what they are telling each other.
I've had several broody hens bring their chickens off of the nest in less than 24 hours after the first one hatched. I had one that did not bring her chicks off until 80 hours after the first one hatched. I was beginning to wonder but they were all fine. I do not feed or water them while they are in the nest to encourage the hen to bring them off. I'd agree, on average it is around 36 hours but each one can be quite different.
A word of warning. I once killed a chick by picking up a broody hen. Sometimes the chicks crawl up under her wings. I crushed one up there when I picked her up. That was a long time ago but those memories tend to stick with you. I still pick a broody up if I need to but I'm pretty careful when I do.
