The cooking time for a rib roast varies, depending on the size of the roast and the doneness desired. Cooking times can vary from 1.25 hours to 3.75 hours.
A rib roast should be cooked initially at 450 degrees Fahrenheit for 15 minutes. Then the temperature of the oven should be lowered to 325 degrees for the remainder of the cooking time. A rib roast is best served rare but can be cooked until it is well done.
A small roast, weighing between 7 and 8 lbs., should be cooked at 325 degrees for 1.25 to 1.5 hours for rare. A roast weighing 9 to 10 lbs. should cook for 1.5 to 2 hours and a larger roast, from 14 to 16 lbs., should cook from 2.75 to 3 hours.