How long does a plastic bag take to biodegrade?

Plastics in general are not considered biodegradable due to their molecular stability. Plastics do not easily break down into simpler components.

A plastic bag will break down into smaller pieces of plastic, but that is not the same as biodegrading.

Some estimates are 500 years to degrade, some more conservative are 1000 years. The bags will photodegrade, but that will take a long time, hence the 500-1000 year estimate. The problem is there are no bacteria breaking down the plastic. Eventually some may evolve to do so, but science may need to step in and speed the process along.