Well if your asking how long it takes to hatch a chicken egg, it's 21 days. A hen must go "broody" to hatch out eggs which means she stays in the nest box all day and she will growl at you if you take eggs away from her. There is no way to force the hen to sit on eggs, you can however give her a pile of fake eggs or golfballs in the nest box. She might decide she wants to sit on them after she sees them laying there. If she does decide to sit on them, let her stay there for a couple of days. She will only get up to eat/drink/poop and then she will usually head back to the nest after a couple minutes. Once you are SURE she is determined to sit on eggs, you can replace the fake eggs with fertilized eggs. She will probably bite you and make angry noises but will settle down once you leave her alone. Wait 21 days and you will probably have cute little chicks running around.
Anyone else, please feel free to add anything I missed!