A good method for determining how long your turkey will take to cook is to allow 12 minutes per pound of turkey once the oven temperature decreases to 325F. For a 12-pound turkey, it should take about 2 hours to roast at 325F. If you have a larger bird, do the math with the weight times 12 to get your approximate cook time. Your turkey may cook faster than that, so check the temperature about 15 minutes earlier than you think the turkey might be done.