What is decomposition of plastic and paper bags?

Plastic does not decompose (at least not in the duration of many, many lifetimes.) Paper decomposes very slowly and depending on the type of paper may take anywhere from one to twenty years to do so. This is because plastic is made from oil which technically still means it is an organic substance however it takes millions of years and very specific conditions for oil to be formed from anything organic. Paper on the other hand comes from trees and will decompose much faster, but if you've ever seen a rotting tree stump you will know it will take it's time to do so still.