Plastic bags have only been around for about 50 years, so there’s no firsthand evidence of their decomposition rate, but the time to degrade a traditional plastic bag represents an estimate anything between 100 and 500 years to dissolve.
Basically, Plastic shopping bags are made of polyethylene, a man-made polymer that microorganisms don’t recognize as food. It is a polymer consisting of long chains of the monomer ethylene (IUPAC name ethene).
In 2007 San Francisco has become the first city in North America to ban the use of traditional plastic grocery bags.
Try to re-use and recycle shopping bags.