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

500 degrees F. first 30 minutes then reduce temp. to 325 degrees F. and roast 20 minutes per pound. Will be 4 and 1/2 hours approx. in total.

This is a response to the above recommendation of 4 and 1/2 hours:

It says to roast the turkey for 20 minutes per pound, then says 4 and 1/2 hours in total. Wouldn't a 21-pound turkey take 7 hours (21 pounds times 20 minutes equals 420 minutes, which is 7 hours)?