How Long to Cook a Prime Rib?

A prime rib roast will take anywhere from 1 1/2 hours up to 3 hours, depending on the size of cut. A small 3-rib, 7 to 8 lb., cut will need to sear at 450 degrees for 15 minutes. The temperature should be reduced to 325 degrees for 1 1/4 to 1 1/2 hours. The internal temperature should reach 120 degrees, or rare.