A 12 pound prime rib will take approximately 3 - 3 1/2 hours (17 minutes per pound) total to roast at a combination of high heat / low heat method. For a mostly medium rare roast, cook until the internal temperaure reaches 125 degrees, for a mostly medium roast, cook until the internal temperature reaches 135 degrees.
Follow these links for great step-by-step directions on preparing your prime rib.