If they have fertile eggs to sit on they will sit until all/most of the eggs hatch. That's 21 - 23 days, usually.
If they don't have fertile eggs I've heard people here say that their hens have sat for over two months!!
If you don't want her to hatch chicks I would suggest trying to break her of her broodiness. The suggestion that seems to work best is to put her in a wire dog type crate with no nesting materials, just food and water. Usually (that's the key word here), she'll stop her broodiness after a few days or a week.