How long do you cook a 18 lb turkey at 350?

For an 18-pound turkey, you should cook it at 350°F for approximately 4 to 4.5 hours if it's unstuffed, and about 4.5 to 5 hours if it's stuffed. It's important to check the internal temperature, which should reach 165°F in the thickest part of the breast and innermost part of the thigh. Let the turkey rest for 20-30 minutes before carving to allow the juices to redistribute.