Internal Temperature for prime rib when fully cooked should be between 120 F and 140 F
Cooking Prime Ribs in the oven at 350 F takes around 30 minutes per pound of the prime rib.
So if you have a 4 lb prime rib it should take 120 minutes to fully cook the prime rib or 2 hours.
The prime rib will be done after cooking for 2 hours at 350 F and when the internal temperature of the prime rib is 120 F to 140 F.