Hypoxia occurs within a few minutes if the cabin pressure altitude rises to between 5,000-6,000 m (about 16,000 - 20,000 ft). Acute hypoxia is characterised by impaired cognitive performance and sometimes a loss of consciousness.
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For best protection, you are encouraged to use supplemental oxygen above 10,000 feet MSL. At night, because vision is particularly sensitive to diminished oxygen, a prudent rule is to use supplemental oxygen when flying above 6,000 feet MSL. So, when you fly at high altitudes, supplemental oxygen is the only solution.
Gradual and progressive onset during flight above 10,000 feet altitude in the absence of normal pressurisation. This can arise either by climb above 10,000 feet without the pressurisation system functioning, or because of pressurisation system malfunction.