Calculate the proper turkey cooking time by taking into account the weight of the bird and whether it is thawed or fresh. If the bird is defrosted, multiply the turkey's weight in pounds by 20 minutes. For instance, a 15-pound turkey should cook for 300 minutes, or five hours.
A fresh turkey should cook in the oven for 10 to 15 minutes per pound, so a fresh 15-pound turkey should cook for between 150 minutes, or 2 1/2 hours, and 225 minutes, or 3 3/4 hours. Determine when the turkey is ready by placing an instant-read thermometer into the turkey's thigh. Remove the turkey from the oven when the thermometer reads 165 degrees Fahrenheit.