At what altitude will a plane's cabin not depressurize?

The Southwest flight that had to make an emergency landing was at 36000 feet, and obviously the cabin started to depressurize. The pilot lowered the plane to 11000 feet, so I'm assuming at that altitude the pressure was equalized. I'm just curious as to what the maximum altitude a plane could fly is without having the cabin depressurize should a rupture appear like the one on the Southwest flight.