The time it takes for plastic to degrade depends on the type of plastic - of which there are 7 main kinds -and the environmental conditions those plastics are exposed to.
Plastics never decompose in the way that plant-based products do (i.e. eaten away by bacterias) and only really break-down thru "photo-degradation", the process by which chemical bonds are broken down by photons in the sun’s light spectrum.
Meaning that plastic only really breaks down under sunlight. This is a major problem because plastic often ends up buried in landfills -in the dark -where it will remain for thousands, if not millions of years.
Seven (7) Main Kinds of Plastic
1. PET: This is the easiest plastic to recycle. Add to it that these materials are relatively cheap and you have the perfect container for soda bottles, water bottles, clamshell packaging, potato chip bags, produce bags and several other grocery-type packaging. Actual breakdown time for this type of plastic under perfect conditions (i.e. under direct sunlight) can range from 5 to 10 years. Buried under landfill, it can take thousands of years.
2. HDPE: This is probably the most recognized recyclable plastic and is used to make detergent bottles, bleach bottles, milk cartons, shampoo and conditioner bottles, motor oil and many other non-food items. Under sunlight, these plastics will degrade in just under 100 years depending on the thickness of the plastic used, but in landfills, it will again take thousands of years.
3. PVC: PVC can be found in a number of items from pipes to children’s toys. PVC does not readily degrade and when it does it gives off a number of toxic materials. This is the single worst plastic according to several health organizations. Greenpeace has been lobbying to stop the use of PVC because of the dioxin produced during its manufacture.
4. LDPE: This is what plastic grocery bags are made of. If exposed to ultra violet light, these bags have been estimated to break down in as little as 500 years with a conservative average time of 1000 years. If there is no exposure to a light source, say at the bottom of a landfill, the plastic may remain intact indefinitely.
5. PP: This material is generally found in more permanent capacities such as rope and clothing made from synthetics (i.e. nylon / polyester). It is highly resistant to photo degradation and will not decay for millennia.
6. PS: This is the plastic in Styrofoam packaging peanuts, cups, coolers and many other lightweight applications. This type of plastic will break down in under 50 years. The surface area exposed to sunlight will determine the exact rate as the more surface area will mean more photons reaching the plastic and a quicker photo degradation process.
7. Other Types: This includes anything not in codes one through six. Most plastics in this section do not breakdown and are considered permanently, chemically bonded.