Ignore the fact that we’d all starve to death before needing to concern ourselves with the oxygen supply. Pretend we and all the animal life magically have an unlimited supply of food on hand. How long until we concert all the oxygen into a lethal amount of carbon dioxide?
You mean convert? 
Boy, beats the hell out of me. I love being super-helpful.
Maybe…
average capacity of a human lung is 5.8 liters.
six billion people = 34,800,000,000 liters of air per breath, and at average of 12 breaths per minute = 417,600,000,000 liters of air “lost” per minute.
the surface area of the earth is 197000000 square miles. assuming a breathable atmosphere 16 miles thick (i think this is an arbitrary number, but what the hey)
=3152000000 cubic miles.
a cubic mile is 147197952000 cubic feet
a cubic foot is 28.32 liters
there are 4,168,646,000,640 liters of air to breathe (obviously i am not breaking this down into oxygen/nitrogen/etc)
therefore, we have 9.98 minutes of air remaining? someone check my math. i suck at math.
i can’t believe i just did that. 
and i forgot about the animals. shit.
And that we don’t take all the oxygen from every lungfull of air. And that we din’t approach full lung capacity except during rigorous excercise.
Or that some oxygen is lost into space. 
Ok, figure average flux of a single breath at .5 liters (I vaguely remember this number from a martial arts class several years ago. Better numbers are welcome). Using CynicalGabe’s other numbres, we get about 115 minutes of air remaining at any particular time. Which is a much smaller number than I expected.
Possible sources of error include:
- Animals breathe too.
- Each breath probably doesn’t convert all .5 liters of air into non-breathable gases.
- Loss of atmosphere into space.
My Google-fu is weak, and my attempts to acquire those numbers were all painful failures. What does the Duck Duck Goose signal look like?
I think I remember reading that each breath actually only consumes about ten percent of the oxygen in the air that’s inhaled. This is why if you seal your head in a plastic bag (kids, don’t try this at home) you won’t have any discomfort for the first few breaths.
/pulls at collar
tough crowd. tough crowd.
ITYM 13,139,572,194,017,300,000,000 litres (3152000000 * 147197952000 * 28.32).
This means 2,189,928,699,003 litres of air per person (for 6 bn humans)
20.9% of this is oxygen i.e. 457,695,098,092 litres.
If each one of us and our supporting biomass (fires, animals etc) consume a litre of oxygen a second, that’s 14.5 millennia.
However, we require a certain percentage of oxygen - 17% if I recall my last fire safety lecture - so we’ve only got a quarter of that: 3.6 millennia.
Are you including marine plants, because I think that’s where most of the oxygen is being manufactured.
As a side note, we wouldn’t suffocate if all of the Amazon rainforrest were cut down tomorrow, the problem with that is how much stuff they burn, not that they’re taking away leaves and leaving us short on breathable air.
For anyone who wants to tackle it, this diagram will give you much of the necessary information.
Without attempting to work through the many complexities involve, I would just note that the diagram allows a very rough estimation of the order of magnitude of the time involved.
Total respiration of autotrophs (animals, non-photosynthetic bacteria etc) currently approximately totals net production of oxygen by plants (oxygen production minus the plants’ own respiration), at about 4.6 x 10[sup]15[/sup] moles O[sub]2[/sub] per year.
The total reservior of oxygen in the atmosphere is about 37,000 x 10[sup]15[/sup] moles O[sub]2[/sub] per year.
Therefore, it will take roughly 8,000 years for all the oxygen in the atmosphere to be used up entirely, without taking any other factors into account. (This also ignores the fact that the oxygen will fall to such a low concentration that it will become impossible to extract considerably before this time.)
Don’t forget the consumption of oxygen thanks to uncontrolled combustion (fires), controlled combustion (keeping people warming and producing other forms of energy), chemical interactions beyond our control, etc.
Duckster beat me to it.
Oxygen is second only to the Halogens in chemical reactivity.
Are we allowed tp extract pxygen from other sources, like water? And presumably there’s a fair amount of it tied up in soils and minerals. Not that this makes me any more capable of answering the OP, but it would need to be accounted for in calculations.
Okay, so we’ve established the figure is somewhere between ten minutes and fourteen thousand years.
On a planetary timescale, that difference is just an eyeblink.
Well, I was close.
But on the eyeblink scale, that difference is one hell of a long blink.
Yeah, what’s nine orders of magnitude in the grand scheme of things?