Last year, or was it the year before, we were lucky enough to have Imogen Napper come and talk to us at Climate Week about the work she is doing at Plymouth University. She told us that one of the things she had started investigating was how fast biodegradable plastic bags actually took to degrade. This has been running for 3 years now and a report is out here. Other, longer term, research is also available from the same page.
Apparently the claim of degradability is a long term one in some environments!