I believe I saw a reference recently saying it only took 40million years before cellulose became widespread biodegradable. I can't find a source for that number, though it looks like it might have been an MS mycology student circa 2014.
I suspect plastic will take significantly less time than cellulose though. It's an amazing material, and the biodiversity of the planet (though shrinking) is still much higher than it was when cellulose began proliferating.
We will almost certainly crisper such beasts into existence and all be replaced with organisms with plastic infrastructure which existing live can't metabolize but thinks our collections of atoms are yummy building blocks.
Think about how often you replace your current laptop.
Even if you are an outlier in terms of keeping tech a long time, upgrading internal components etc, there is very little chance rotting plastic will be the reason you laptop fails.
I've been wondering if wood or some kind of injection molded cellulose product might make a comeback. Decomposing plastics would probably help spur that along.
http://hn.premii.com/#/article/15977640
I suspect plastic will take significantly less time than cellulose though. It's an amazing material, and the biodiversity of the planet (though shrinking) is still much higher than it was when cellulose began proliferating.