Plastic lasts hundreds or thousands of years in the environment.
The first thing to say is that all organic materials degrade (namely everything based on carbon). Standard plastics degrade outdoors in a few years, or even sooner unless they are stabilized. But, if put in landfills, even food and paper don’t degrade because there’s not enough oxygen present.
Sources: Chris DeArmitt, "The Plastic Paradox - Facts for a better future"
Dr. William Rathje, "Rubbish!: The Archaeology of Garbage"