Time is certainly a factor.
Any organic compound is subject to oxidation under proper conditions.
It is assumed that if organic material was ever there it has long since been oxidized out. A standard procedure for releasing pollen and spores from coal macerals is to treat them with a mixture of concentrated nitric acid saturated with potassium permangenate. I cannot imagine a stronger oxidizer than that. This stuff completely oxidizes the carbonaceos material in the coal leaving essentially sporopollenin as the surviving organic. However, the time of exposure of the coal to this oxidizer must be optimized to prevent loss of spores and pollen, which will certainly happen with continued exposure.