Prime Ribs take around 15 to 25 minutes per each pound of the prime rib to cook.
So if your Prime Ribs are say 4 lbs then you would cook the prime ribs for 60 minutes to 100 minutes.
When fully cooked the internal temperature of the prime rib should be at least 120 F.
As long as the prime rib has been cooked for at least 100 minutes then it will be fully cooked and ready to eat once you let it sit and cool off.