Your boneless prime rib cooking time will depend on the size of roast you're making: 4 lb. roast: 15 minute sear + 1 hour 40 minutes cook time 5 lb. roast: 15 minute sear + 1 hour 45 minutes cook time 6 lb. roast: 15 minute sear + 1 hour 50 minutes cook time
The ideal cooking temperatures for a prime rib roast are 325°F and 10-15 minutes per pound. However, you may even prepare prime rib roasts at temperatures as low as 200°F, but keep in mind that the meat will take a long time to cook through fully.
You can cook it for about 25 mins per pound, so this means if you are cooking 2 ribs weighing around 4 pounds then it should take about 1 hour and 40 mins approx. You can always check it 30 mins before the estimated time. If you want it medium rare you can check the temperature if it reaches 130 degrees then you would know it’s cooked medium-rare.
“The Slow Roast Method for roasting prime rib is easy, hands-off, and foolproof. Roasting at a low temp ensures that the meat cooks evenly throughout, rather than with a well done exterior and a rare interior. This method also leaves you time to get everything else ready while the meat slowly cooks to perfection.” — Joan Velush