(Polythene is degraded fairly…)
Plastic bags have only been around since 1957. So, this is obviously just an estimate. Polythene is degraded fairly effectively by ultra-violet light, but plastic bags in landfill are quickly buried and so don't get much light. Bacteria and fungi have a much harder time digesting it. Polythene doesn't exist in nature so soil microorganisms that are well adapted to breaking it down haven't yet had time to evolve.
In 2008, Daniel Burd, a high-school student from Canada, found that a brew of Pseudomonas and Sphingomonas bacteria, together with yeast and sodium acetate, could digest 43% of a plastic bag in six weeks. This culture needs to be kept at 37° C (99° F), so it would probably need to be used in an industrial reprocessing plant rather than an ordinary landfill.
BBC Knowledge Magazine
October Issue On Stands