How much of earth's molecular oxygen in the atmosphere is due to plants?
The oxygen in the atmosphere was produced by cyanobacteria during the Great Oyxgenation Event, around 2.3 billion years ago. Cyanobacteria produced &amp; still, produces, to a lesser extent, oxygen by photosynthesis. Plants did not exist during the Great Oxygenation Event, but these day plants replenish atmospheric oxygen by removing carbon dioxide by photosysnthesis, like cyanobacteria. According to David Biello, The Origin of Oxygen in the Earth's Atmosphere the production of oxygen by cyanobacteria during the Great Oxygenation Event took one billion years.

During the Carboniferous period, about 350 million years ago, the amount of oxygen in the atmosphere increased to 30% because the rate of burial of organic matter prevented it from reacting with atmospheric oxygen, History &amp; Significance of Oxygen in the Atmosphere

According to Wallace S. Broecker our atmospheric oxygen supplies are vast: 


  Simply put, our atmosphere is endowed with such an enormous reserve of this gas that even if we were to burn all our fossil fuel reserves, all our trees, and all the organic matter stored in soils, we would use up only a few percent of the available $\ce{O2}$. No matter how foolishly we treat our environmental heritage, we simply don't have the capacity to put more than a small dent in our $\ce{O2}$ supply. 


From Atmospheric oxygen - Broecker