Hens should stay on their egg most of the day no matter the weather. In very hot weather sometimes they will stand over their eggs, but the instinct to stay on their nest is strong. They typically take a break midday, have a toilet break, eat some food, drink some water, maybe dust bathe, but then it's straight back to the eggs. In the last few days before the eggs hatch they won't move off their nest. Instinct tells them what to do.