How long to cook a 20 lb turkey has a very simple answer but it would not be consistent in every scenario. What kind of stuffing is being used, the quantity of the stuffing, if there is any stuffing at all, whether the skin is on or has been taken off, the present condition of the turkey in regards to being fresh, thawed or frozen; all of these factors would determine or at least influence to varying degrees the time it would take to cook a 20 lb turkey.
Cooking Quick Guide
Here is a quick guide on the time it takes to cook turkeys of various sizes and weights.
1. Considering the temperature at which the turkey is being cooked to be set at 325 degrees F and assuming that the turkey has already been thawed or is fresh, the time taken to cook a 20 lb turkey is four and a half to five hours. This is for unstuffed turkey. If the turkey is stuffed, then the time needed would be five hours at least. It could very well be five and a half hours or a tad more.
2. The typical approach is, for unstuffed turkeys, cook for up to three hours for 8 lb to 12 lb of turkey, about four hours for 14 lb to 18 lb of turkey and four and a half hours for 18 lb to 20 lb of turkey. For stuffed turkeys, cook for about three and a half hours if it weighs 8 lb to 12 lb, about three and a half to four hours for 12 lb to 14 lb of turkey, four and a half hours for 14 lb to 18 lb and close to five hours for 18 lb to 20 lb.
3. The aforementioned cooking times do not include the prepping time, the time it takes to thaw the turkey and the time needed to cook the stuffing. The specific time needed may vary as the quality of the turkey meat, the tenderness or rigidity of the flesh, the efficacy of the cooking mediums and the nature of the stuffing; all would play a role. What you should look for is an internal temperature of 165 degrees F. That is the minimum reading you should look for. Turkey meat measuring any lower on a food thermometer is not safe or ready to be eaten.