When breathing, our lungs only reduce the oxygen content in air from  21% to 15%. This means the total oxygen consumption per average human in a day is about 660 litres, weighing less than a kilogram. That doesn’t seem like an awful much, but we can't really judge without knowing how much is produced.

Let’s compare human oxygen consumption to the oxygen output of a fairly large tree of 50 cm diameter and approximate height of 30-40 metres. This tree produces roughly 123 grams or 92 litres of pure oxygen a day, providing the average human with 14% of his or her daily requirement. That means an average human could survive in an air-tight space if it was co-habited by 7 very large trees.

What about the 7 billion people besides me?

Our global population would require the equivalent of 49 billion trees to sustain the oxygen levels humans required. So, are there enough trees for that? NASA has made an approximation of the number of trees on Earth based on satellite images and their guess is the world currently holds about 400 billion trees, which is plenty more than needed. 

However, keep in mind that humans are not the only air-consumers on Earth. Besides other wildlife, natural and artificial fires (think internal combustion engine) are major consumers of oxygen. In fact, it is claimed that the oxygen content in our atmosphere is declining at 4 ppm per year, which equals a loss of 0.00002% oxygen per year.

Luckily, the amount of oxygen already stored in the atmosphere is very large and 90% of all living biomass on Earth are oxygen-producing plant matter, whereas most of our oxygen comes from deforestation-proof oceans. Our oxygen reserves are so large, in fact, that if photosynthesis suddenly stopped and all 7 billion people were stuck on our planet with no other life forms and no fire, it would take about 50 million years to breathe up all the oxygen our atmosphere has stored. 

If we considered all annual oxygen losses due to combustion and respiration by all living matter, our oxygen storage would last for less than 2000 years. All in all, oxygen levels are dropping at a very small rate, but luckily we still have plenty of time to correct it. That's if we get enough oxygen to the brain before we run out of it!