The length of time required to cook a prime rib roast depends on the size and doneness. The browning period takes 15 minutes at 450 F followed by slow roasting at 325 F. Total cooking time ranges from 1 1/2 to 4 hours to achieve rare doneness.
Prime rib also needs to rest, covered in foil, for 15 to 30 minutes after it is removed from the oven to finish cooking and redistribute the flavor and juices. The roast temperature will rise by 5 to 10 degrees as it rests. Doneness is determined by internal roast temperature, using a meat thermometer inserted into the center of the roast. The doneness temperature ranges from 125 F for rare to 160 F for well-done meats.