Prime Rib roast takes 15 minutes per lb to fully cook in the oven at 325 F degrees.
So if you have a 5 lb prime rib roast then you'd cook the 5 lb prime rib roast for 75 minutes which is a little over an hour.
Just adjust the time to the lb of your prime rib for the correct cooking time which is 15 minutes per lb.
The internal temperature of your prime rib should read 120 F degrees when the prime rib is fully cooked and ready to serve.