I would candle if she has been on it for more then 21-22 days. If you intend to give her chicks or more eggs to hatch, then just leave her set (But it is a good idea to provide her food and water and separate her). You could stick some plastic Easter eggs or golf balls under her while you find fertile eggs or chicks to put under her.
Otherwise, I would break her up so she doesn't weaken herself (even if you provide feed and water, she still will grow skinny. It takes a lot of energy to set like a broody hen.). To break her up, put her in a cage or pen with a wire floor, or I use upside-down rabbit cages, set up a few inches. This way the air can drift under her at night and during the day. In about a week of this she should be back to normal, though some may be quicker or take longer. And provide her food and plenty of water!
edt: And yes, she will set for weeks or months! I had a hen sit for seven months!