Biodegradation is a process when a substance or an object is being decomposed by bacteria or other living organisms. Therefore plastics don’t really “biodegrade” as bacteria are not “interested” in them. This means that plastics can’t be converted into useful compounds for life. They photodegrade instead. It means that photons from the sun pulverise the plastic polymers until they are broken into individual molecules. But even then they remain PLASTICS!
This makes them even more dangerous for us as they can easily enter the food chain.
For example a plastic bottle will degrade in this way in around 500 years (up to a 1000 years).
Recently the scientists discovered a mutated bacteria that produces an enzyme called PETase which can break down PET (Polyethylene Terephtalate) mainly used to produce plastic bottles. It means (to my understanding) that this particular type of plastic could be recycled in a much easier way but it doesn’t mean it would completely “biodegrade”.
Therefore an answer to the question “how long does it take to plastics to biodegrade” is: never.
Ransford M, “Why Trashing the Oceans Is More Dangerous Than We Imagined” (Popsci.com, 2008)