The rule of thumb is no food should be left out of refrigeration longer than two hours. To be on the safe side, I refrigerate everything after one hour. After about 45 minutes the meat will start to grow bacteria. Assuming that the ribs are cooked all the way through, 45-1hr would be the longest i would let them sit. If they're not cooked all the way through and /or there is any blood congealed, I would refrigerate them sooner than that. (better safe than sorry).