Prime Ribs take around 15 to 25 minutes per each pound of the prime rib to cook but the time it takes to fully cook prime rib depends on how many lbs of prime rib you're cooking.
So if your Prime Ribs are say 4 lbs then you would cook the prime ribs for 60 minutes to 100 minutes or if you don't think it's done enough at 100 minutes you can let the prime rib cook for 2 hours or 120 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 to 120 minutes then it will be fully cooked and ready to eat once you let it sit and cool off.