If you’re cooking your prime rib at a higher temperature, like 325°F, you’ll need to decrease the cooking time. For a 3-bone roast, you should cook it for approximately 2½ hours. For a 4-bone roast, cook time will be closer to 3½ hours. And for a 5-bone roast, you should plan on cooking for approximately 4½ hours.
Prime rib roasts are often measured by the number of rib bones you get in your cut. The full roast, usually called a "standing rib roast" when it's sold that way, has seven ribs. Usually, they're cut into smaller, more practical sizes for retail. A two-rib roast will weigh 4 to 6 pounds, while a four- or five-rib roast can weigh 8 to 10 pounds.
Well, this depends on the weight of the rib. You can cook it for about 25 mins per pound, so this means if you are cooking 2 ribs weighing around 4 pounds then it should take about 1 hour and 40 mins approx. You can always check it 30 mins before the estimated time.
This calculator is very similar to our Beef Rump or Round Roast Calculator, however the cooking times vary for a rib roast (especially if it is bone in or boneless). The calculator will provide you with cooking times for both types.