This is why I try my best to wear organic fibers. If I can't donate them at least they'll quickly decay in a landfill. A quick google search revealed that synthetic fibers, polyester, spandex, nylon, take 20 to 200 years to decompose.
That also means you can wear them forever though. You could probably wear out 3 cotton items for every one synthetic.
Synthetic clothes shed microplastic particles in the wash. It's actually a significant source of plastic pollution. Cotton is kind of crap fabric and takes enormous amounts of water to grow, but wool and linen are both much more durable.