Cooking in a brown paper bag will actually shorten the cook time rather than extend it, although only very slightly. The paper bag will trap some, but not all, moisture during cooking and make steam, which cooks faster than dry heat. It is also said to trap in flavor that may have escaped during evaporation. A closed roasting pan or dutch oven will do the trick better than the paper bag as it allows no steam to escape. One thing to keep in mind when using this method of cooking is that steam prevents the skin from caramelizing, so do not use this method if you are looking for crispy skin. You can lift the lid for the last 20 minutes or so to crisp the skin a little, but it will not be as pronounced as when using other cooking methods. To slow down the cooking time of a 12 lb turkey, you should cook at a lower temperature. The lowest I would recommend is 165 degrees, as any lower will leave the turkey at bacteria growing temperature too long. If your oven does not go that low, as most home ovens don't, cook at the lowest available (often times 225 or 250). This will extend the cooking time but probably not overnight (8hours). It should also be noted that one should not sleep with the oven on over night for safety reasons. The question does not explicitly state that as the intention, but i fear it is implied.
The safe cooking temperature for turkey is 165F (74C).
The safe temperature for cooking a turkey is 165F (74C).
The minimum safe cooking temperature for a turkey is 165F (74C).
The recommended cooking temperature for roasting a turkey is 325F (163C).
The recommended temperature for cooking turkey meatloaf is 165F (74C).
The recommended turkey fryer temperature for safely cooking a turkey is 350 degrees Fahrenheit.
The recommended cooking temperature for ground turkey meatloaf is 165F.
The recommended turkey fry temperature for safely cooking a Thanksgiving turkey is 350 degrees Fahrenheit.
The recommended internal temperature for a spatchcock turkey when cooking it is 165F (74C).
The recommended internal temperature for cooking a stuffed turkey is 165F (74C).
The minimum temperature that should be reached when cooking a turkey is 165 degrees Fahrenheit.
The recommended grill temperature for cooking a delicious turkey burger is 165F.