Biodegradable plastics take three to six months to decompose fully. That’s much quicker than synthetic counterparts that take several hundred years. Exactly how long a biodegradable bag takes to break down depends on various factors, such as temperature and the amount of moisture present.
But the bags aren’t always as environmentally friendly as they seem. They’re made from similar petrochemical-based materials to conventional plastic, only with compounds added that cause them to disintegrate gradually in the presence of light or oxygen. They often then degrade into a sludge of toxic chemicals.
Bioplastics made of cornstarch and other plant-based materials are a better bet. They give off CO2 as they decompose, but they’re merely expelling carbon locked in by the plant matter that originally formed them. The net effect on the environment is therefore close to zero.