Plastics are basically forever. They just break down into smaller and smaller pieces and contaminate the environment (they've found commercially sold sea salt contaminated with micro-plastics).
With the exception of the plastics that are biodegradable (expensive and extremely uncommon) and the ones that we've discovered worms can eat.
I used to think that. Then I read this and now I'm less sure:
....the most widely-used plastic, the stuff used to make shopping bags, is the one that produces the greatest amount of these warming gases ... after 212 days in the sun, this plastic emitted 176 times more methane than at the start of the experiment. Ironically, when plastics were exposed to air the amount of methane emitted was double the level from sea water.
more here: https://www.bbc.com/news/science-environment-45043989
But it's better than sticking it in a landfill and waiting 1000 years. Or worse, tossing it into the ocean.