It takes 21 days for eggs to hatch, so right there is 3 weeks. Since hens dont have a calendar near by, 21 days isnt an automaic thing even if they dont have eggs. Ive had hens sit for about a month!! Ive tried everything, but havent had anyh luck. Silkies and Cochins just live to be mammas!!
A broody hen will only come off the nest once a day to eat, drink and poo. Other then that she just sits there looking half asleep. Some things you can try to break her are pulling her off the nest constantly. Giving them treats once you throw her off. Try dipping her in water (make sure its warm enough!!) Try placing ice cubes in her nest. or placing her in a wire bottom cage where air can circulate below her.
Ive never had any luck, but you may. Btw, what kind of hen is she?
Good luck,
Shannon