Interesting fact: At 40,000 feet, the air is so lacking in oxygen, that if you were to breathe it normally, you would have around 12 seconds before starting to suffer the effects - disorientation, lack of consciousness, and eventually death.
All aircraft have their own guidlines for descent rates should their be a loss of pressure inflight, this varies from aircraft to aircraft, but it is roughly around 6000 feet per minute descent for a B737.