Cooking a Turkey at 325 F degrees takes around 15 minutes per lb of turkey so for your 12 lb Turkey it will take 180 minutes or 3 hours for the 12 lb Turkey to be fully cooked.
The Turkey will be fully cooked when the internal temperature of the Turkey is at least 165 F but if you prefer it more done then you can continue to cook the Turkey until it reaches 180 F.
So to cook a 12 lb Turkey at 325 F degrees it takes 180 minutes which is 3 hours.