Interestingly, the "biodegradable" bags seem kind of scam-ish. Apparently the PLA (corn based) ones only break down in a "commercial" composting environment. According to this article, that means 10 days at some specific temperature (the article says "70 degrees" but no more).

http://www.smithsonianmag.com/science-nature/corn-plastic-to...

Only cottoned onto this personally, after noticing the "compostable" bags I'd been using to hold vegetable scraps... weren't compositing in the home compost bin even many months later (nearly a year now). :/