It takes up to four hours to cook a prime rib roast on a rotisserie, assuming the roast weighs 4 to 5 pounds. This cooking time varies with the style of grill and the location of the rotisserie in relation to the heat source. The roast cooks faster the closer it is to the source of heat.
After three hours, the internal temperature of the roast must be tested, taking care to insert the meat thermometer away from the rotisserie rod. For a medium-rare roast, the beef is removed when it reaches 120 degrees Fahrenheit, and it is allowed to rest until the temperature rises to 130 degrees Fahrenheit.