Some stop a few days before the hatch, others get off to eat and drink while the chicks are hatching! It depends on the hen, how hungry or thirsty they are, if they are scared or pushed off (such as by another hen) and if it is warm or cold (they get off more often and for longer during warm...
When I let my hens raise their chicks in the main flock (from day one) I generally loose at least one, sometimes more, often in horrific ways. : (
Usually I wait at least three days until they are strong enough to keep up well, other times I wait up to a month!
Some hens are just nasty...