The rule of thumb is no more than four hours between 40F and 140F. That said, the primary contamination risk for whole muscle meat, such as a rib roast, is the surface of the meat. The interior of the meat would be virtually contaminant free. Since the surface of the meat would rise above 140 pretty quickly, many would consider the food safety risk of long cook times to be minimal with whole muscle meats.
Ground meats have different risks because the grinding process distributes any surface contaminants throughout the meat. That's why ground meat is considered safely cooked at about 152 degrees while a rare steak is considered safe at 125 degrees.