Different kinds of plastic can degrade at different times. In sum, as time passes, we know that plastic will eventually photo-degrade, i.e. break down into smaller and smaller fragments by exposure to the sun.
The photo-degradation process continues down to the molecular level, yet photo-degraded plastic remains a polymer. No matter how small the pieces, they are still and always will be plastic, i.e. they are not absorbed into or changed by natural processes.
At sea, the plastic fragmentation process occurs as well, due to wave, sand action, and oxidation. Estimates for plastic degradation at sea has been ranged from 450 to 1,000 years.https://plastic-pollution.org/
The average time for a plastic bottle to completely degrade is at least 450 years in most plastic types which is 164 250 days! That’s a really-really long time.