1 20 to 24 lbs: 4 1/2 to 5 hrs. 2  When you remove it from the oven, cover it very loosely with foil, and go about getting your side dishes to the table (or have a glass of wine and delegate). 3  Don't worry, it won't get cold; a covered 20-lb turkey will stay hot for over 40 minutes, so don't rush it. Remove the turkey when it reads 165 degrees F. in the thickest part of the thigh meat. 2  Here is an approximate turkey cooking time guide for roasting at 325 degrees F. from the USDA: 3  8 to 12 lbs: 2 3/4 to 3 hrs. 4  12 to 14 lbs: 3 to 3 3/4 hrs.