Plastic bags in generally usually don’t decompose, like uranium. It takes about an average of 300-500 years to decompose plastic. Best way the decompose plastic is to melt it, and then make it into something else.
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Left on the surface of the earth, with sun, air, water attacking it it can decompose in months. Buried it can take a long long time, centuries. But I would not try to use it after a decade.
