Time of Useful Consciousness
This is the time available to an aircrew member to recognize they are suffering from hypoxia and to take appropriate action (put on an oxygen mask and/or descend the aircraft below 10,000 feet).
The Time of Useful Consciousness is a function of altitude. At 20,000 feet, an average individual will have 5 to 12 minutes. At 25,000 feet, this time is reduced to 3 to 5 minutes. At 30,000 feet only 1 to 2 minutes are available. By 40,000 feet, the average individual will have only 9 to 15 seconds (basically this represents the oxygen that was in their system before the exposure to 40,000 feet).