Without human intervention, a leaf takes about two years to decompose naturally. The actual amount of time for decomposition is affected by factors that include heat, the type of leaf and the amount of exposure to oxygen.
Decomposition can be accelerated by shredding or chopping the leaves, which is easily done with a lawn mower, then placing them in a well-ventilated area with plenty of oxygen. Moisture also helps with the process, so leaves should be kept out of direct sunlight. Oak leaves tend to take longer to decompose, while leaves with a higher sugar content disintegrate more quickly than others.