According to Dan Espen :

Every so often a plastic candy wrapper or toy will get into my
compost heap. It's always interesting seeing the wrapper or toy
turn up a few years later still shiny and looking new. Everything
else has turned to dirt but the plastic is in there for the long
haul.

Depends on the plastic, how well it was made, and what it's been
exposed to. Temperature cycling and exposure to harsh cleansers
(ie: chlorine bleach) or UV can cause many plastics to embrittle or
even disintegrate.

Several of the plastic parts of our fridge have gotten rather
fragile. It's over 20 years old, so it's somewhat understandable.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.