Cooking an 18 lb turkey can be a bit of a project! Typically, you'll want to roast it in an oven at 325°F. Allow about 4 to 4.5 hours if it's unstuffed. If you decide to stuff it, which adds time, it might take closer to 4.5 to 5 hours. Remember, using a meat thermometer is key to make sure it reaches a safe internal temperature of 165°F. Do you enjoy preparing big meals, or is this a special occasion for you?