It takes leather between 25 and 50 years to completely decompose. Leather that has been tanned can take significantly longer to decompose because of the chemicals used to tan it.
As with all waste material, the amount of time it takes for leather to decompose may be heavily disputed. However, the general consensus among environmentalists is that leather can take up to 50 years to become part of the soil. Chemically treated leather takes longer to decompose, and thicker leather, such as that used to make shoes, takes longer than thinner pieces.
By comparison, rubber-soled shoes take from 50 to 80 years to decompose, an orange peel takes six months and Styrofoam never decomposes.