Generally speaking, the oven temperature should not be lower than 325 o F. Some turkey recipes use an initial temperature of 500 o F at which a 15-pound turkey is roasted for 30 minutes and then cooked at 350 o F for another 2 to 2.5 hours. But that it not definite. You need to watch the inner temperature of the turkey.
As always, the temperature inside the breast or the stuffing or the coldest spot in the turkey should reach a minimal of 165 o F no matter how long you cook a turkey. If you choose to stuff your turkey, combine the dry and wet ingredients just before you cook them. And then fill the turkey's cavity loosely so that heat can penetrate into the stuffing easily saving some time. That is the USDA advice for the safety concerns.
Un-stuffed
4 to 8 pounds (breast) 1½ to 3¼ hours
8 to 12 pounds 2¾ to 3 hours
12 to 14 pounds 3 to 3¾ hours
14 to 18 pounds 3¾ to 4¼ hours
18 to 20 pounds 4¼ to 4½ hours
20 to 24 pounds 4½ to 5 hours
Stuffed
4 to 6 pounds (breast) Not usually applicable
6 to 8 pounds (breast) 2½ to 3½ hours
8 to 12 pounds 3 to 3½ hours
12 to 14 pounds 3½ to 4 hours
14 to 18 pounds 4 to 4¼ hours
18 to 20 pounds 4¼ to 4¾ hours
20 to 24 pounds 4¾ to 5¼ hours
You may cook frozen turkey directly. But it needs to double the cooking time that is needed for thawed turkey.
Also remember that you should let the roasted turkey covered with foil sit at room temperature for 20 minutes, which is needed to make your carving a little easier.
