How long does it take to cook a 21 pound turkey at 450?

450 degrees is too hot to cook a turkey because it will become dry. If you are cooking it in the oven, follow the directions on the plastic packaging that comes around your turkey. Usually about 350 degrees is better because you want to cook a turkey slow until the fork goes in easily about 5 to 8 hours of cooking time, unless you are deep frying it then that is a different procedure.