If we consider lignin as the first plastic (not man-made), it took about 60 million years. See https://www.nationalgeographic.com/science/article/the-fantastica

If we consider lignin as the first plastic (not man-made), it took about 60 million years. See https://www.nationalgeographic.com/science/article/the-fantastically-strange-origin-of-most-coal-on-earth https://www.nationalgeographic.com/science/article/the-fanta...