Broody hens should always leave the nest at least once a day to eat, poop, dustbath and drink during incubation.
If they don't, there is either something not functioning properly in the broody, or the keeping conditions are wrong. A good broody does not poop in the nest site for obvious reasons.
You may find that a broody hen will stay on the nest for the first couple of days while she sets the eggs and gets them started.
Even in the last three days broody hens here will still leave the nest, even when chicks are starting to hatch.
If she is sat tight now, you should leave her be until she has finished hatching. When that time comes and she leaves the nest, do not be tempted to make her sit on any unhatched or partially hatched eggs.