Every breath you take is about 20% oxygen, while what you exhale is about 15% oxygen and about 5% carbon dioxide. Normal oxygen saturation is 98 - 100% while at 70% saturation; it is considered de-oxygenated. Sustained levels below 90% are dangerous for your health and are considered hypoxic (insufficient saturation). Supplemental oxygen is medically indicated at around this level (at rest).
Active tissue has quite high oxygen demands (4 or more times higher than at rest); as an example if blood flow to the brain is stopped completely, it takes just 12 seconds for that blood to be depleted of oxygen, unconsciousness to occur and cell damage to start.
Without breathing/being able to breathe, most people lose consciousness quite quickly and the medical consensus seems to be that at normal body temperature, brain damage begins within about 4 minutes, long after consciousness is lost. At this point, survival would not be guaranteed even with medical intervention.
People engaged in extreme sports like free diving can hold their breaths for around 2-3 minutes after intense physical and mental training and possibly drugs to improve oxygen carrying capacity but death from asphyxiation is not uncommon in these people.
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