250 for about three hours, but you have to use a probe because the prime rib will cook longer depending on its weight. It's good to keep notes so if you have another roast at the same weight you can guess how long to wait before checking the temp. I usually put a rub on my prime rib and set it on four onions cut in half so the drippings drain off. Save your drippings to make beef and barley soup and the au jus.
