Sometimes they do just take a break, but they dont sit on the eggs in the nest while they are having a break, lol. If she is in there almost all the time then she is probably broody and trying to hatch the eggs. If so, she will might fluff up her feathers and may even growl or peck at you if you try to reach under her for the eggs. Or she just may refuse to get off them. She will frequently rearrange the eggs underneath her. Broody hens will usually get off the nest once or twice a day to eat, drink, and poo (really stinky large saved up kinda poo). If you want her to hatch some eggs, and you can get some fertile eggs from someone else, this would be the time to put them under her. However, the other hen will continue to lay in the nest (broodies stop laying) so you must mark the fertile eggs so that you can remove the fresh infertile egg everyday.
Another possiblilty, and not a good one, is that she may have become an internal layer, or be egg bound. This means that an egg is stuck inside her and she will get sick and die if she cannot pass the egg. If that is the case, she will more than likely exhibit signs of trying to lay an egg. Squatting, getting in and out of the nest repeatedly, seeming in distress. You may even be able to feel a mass in her lower abdomen. Sometimes a warm bath, some gentle abdominal massage, or some lubricant in her vent may help. Often the prognosis is not good.