Numerous international governmental agencies that steer policy assume that polystyrene, a sort of plastic  persists in the environment for millennia. 

In their research paper published in the Journal of  Environmental Science and Technology Letters, scientists show the  that polystyrene is completely photochemically oxidized to carbon dioxide and partially photochemically oxidized to dissolved organic carbon. Lifetimes of complete and partial photochemical oxidation are estimated to occur on centennial and decadal time scales, respectively. These lifetimes are orders of magnitude faster than biological respiration of polystyrene and thus challenge the prevailing assumption that polystyrene persists in the environment for millennia. 

Excerpt from Collin P. Ward et al, Sunlight Converts Polystyrene to Carbon Dioxide and Dissolved Organic Carbon, Journal of Environmental Science and Technology Letters, October 10, 2019