How Long do I Cook a Prime Rib?

The time to cook the perfect prime rib, depends on the size of your ribs and your taste. For a dinner for six you will need three ribs, or about 7 to 8.5 pounds. The ribs will take about 1.5 hours in a 325 degree oven. For medium rare the internal temperature should reach 135 degrees and the meat will be very pink, which is perfect for prime rib.