About how long will it take cook a 20 pound turkey? Every year I go by the guidelines and it always takes longer!!!!

It may depend on how cold your turkey is when you put it in the oven. If it is right out of the fridge it will take longer than the guidelines.

I cook it at 20 minutes per lb. Start at 325F and raise the temperature to 350 for the last hour. It's also a good idea to cover the breast with foil and remove the foil for the last hour.
Works every time for me.

A good rule is 13-15 mins per pound which translates to about 4.5 hours. But if its stuffed it will take longer. And 160° is ideal temp to pull the turkey at. It will carry over to 165°+