[the_seraphim]

Based on their being 10^15 tons of oxygen in our atmosphere and the average human requiring 0.306 tons of oxygen per year to survive there is enough oxygen in the atmosphere for humanity to breath for 4.6 billion years

however, we require a minimum of 19.5% so with current oxygen of 21% we could lose 8.625^10 tons of oxygen maximum

8.625^10 tons of oxygen would last the entire human population of 7 billion people just over 40,000 years

plants make up 80% of the biomass of the planet, humans only 0.01%

it really does depend on the oxygen requirements of the rest of the biomass on the planet though, 15% of that biomass is bacterial... hopefully they dont use as much air as we do otherwise we only have enough for 4 years.