I agree with Flockwatcher. I've never had a broody that starved herself to death, but a few people on this forum have posted they have. I'm not always sure I believe that when I see it. They are living animals and you can never be sure what they will do, but I sometimes suspect something else caused the death. A broody does not eat, drink, or exercise as much as non-broody hens, which can weaken them. It is possible roost mites or an underlying condition killed them in their weakened condition, not necessarily that they starved themselves to death.
When the weather is hot, like the middle of summer here, it is not that unusual for me to see a broody off her nest for an hour or so at a time, maybe even twice a day. If the weather is cool or cold, they seem to hop off, eat, drink, poop, and dust bathe, then hop right back on. There are some broodies that I hardly ever see off the nest.
I don't know if you have isolated her or she is hatching with the flock. If she is with the flock, I suggest you mark the eggs you want her to hatch and check under her daily to remove any other eggs. This is a good time to toss her out of the coop door to see if she will eat, drink, or poop. When I do this, some of mine take advantage of the opportunity for their daily constitutional, but some just run right back on the nest. But by checking daily, you can see if she has pooped in the nest. If she has not pooped in the nest, she is almost certainly getting up.