How long do you roast a 20 lb turkey?

It takes a long time. One day in the fridge for each 5 lbs. A 20 lb. turkey will take 4 days in the refrigerator. If you need to defrost in a hurry you will need to take drastic action and provide a running bath of cold water, not a static bath. Check often and prepare for the oven as soon as possible.

The key here is temperature is not your friend with food. Bacteria will grow between 40 and 160 degrees (they actually never stop growing, even in the freezer but grow much, much slower) The amount of time a turkey (or other food) spends in this danger zone is crucial. So get that turkey into a hot oven as soon as it is defrosted in this manner.