Judge when your turkey is done cooking by taking its temperature rather than by looking at the clock. There are far too many variables for any recipe to be able to accurately predict exactly how long your bird will take. And 5 or 10 degrees is enough to make the difference between a perfect turkey and one that's over- or under-cooked.
What is the proper temperature for a roasted turkey? For years the Department of Agriculture called for 180 degrees, but that was a sure recipe for a tired, overcooked bird. Now the recommendation is a much saner 165 degrees.
But notice that if you're roasting a stuffed turkey, you'll need to take the temperature deep in the center of the stuffing, by which time the breast meat could be overcooked. For that reason, it's usually best to cook the stuffing outside the bird (this will also result in a much shorter cooking time).
Note also that there is usually at least a 10-degree increase in temperature that comes in the 30 minutes between removing the bird from the oven and carving it. That "push" after roasting should result in a final temperature of around 175 degrees.
To take the temperature, insert an instant-read thermometer in the deepest part of the thigh, nearly to the hip joint, but taking care not to touch the bone. This is the area that is always the slowest to cook.
There are other indicators for doneness. Press the fleshy part of the thigh with your fingers; if the meat feels soft, or if the leg moves up and down easily and the hip joint gives readily, the turkey should be done. Doneness can also be determined by inserting a long-tined fork into the thickest area of the inner thigh. If the juices run clear, not pink, the turkey should be done.
While you should always use temperature as the most reliable indicator of doneness, it does help to have a rough timetable for scheduling guidance. According to the USDA, for an unstuffed turkey, you should allow 2 3/4 to 3 hours for an 8- to 12-pound turkey; 3 to 3 3/4 hours for a 12- to 14-pounder; 3 3/4 to 4 1/4 hours for a 14- to 18-pounder; 4 1/4 to 4 1/2 hours for an 18- to 20-pounder and 4 1/2 to 5 hours for an 20- to 24-pounder.
For a stuffed turkey, allow 3 to 3 1/2 hours for an 8- to 12-pound turkey; 3 1/2 to 4 hours for a 12- to 14-pounder; 4 to 4 1/4 hours for a 14- to 18-pounder; 4 1/4 to 4 3/4 hours for an 18- to 20-pounder; and 4 3/4 to 5 1/4 hours for a 20- to 24-pounder.