(first published April 2009, updated September 2010)

Reading that an item takes eleventy snazzillion years to break down in the environment makes for great attention grabbing stuff when writing articles on green living, but the more I research various related topics, the more I find differences in estimations.

A lot of this is to do with the fact that decomposition is very dependent on the environment where the material is decomposing. For example, look at the difference between how fast steel rusts (a form of decomposition) in a humid salt air environment such as the coast vs. a dry environment like a desert. 

Dry air really slows down decomposition generally. Another example is cardboard, something we consider very biodegradable. I have a sheet of cardboard out the back of a shed at my property in the outback (semi-arid area) and after 2 years it still hasn't broken down even though it's fully exposed to the elements.

Another factor is whether the waste is even exposed to the air or buried in a landfill. In the case of the latter, it usually takes a lot longer for an item to break down.

Those are a couple of issues to bear in mind when reading statistics on how X or Y material breaks down. "Break down" is really a vague term and there's also a big difference in the terms, biodegradable, degradable and compostable; not to mention the types of residues they leave behind, some of which can be toxic. It's another good reason to recycle where we can, plus recycling energy savings for most types of waste are significant.

However, we can get a general guesstimate about waste decomposition and that can help in making purchasing decisions.
 
With all that in mind, here's a list of common items and how long they take to "break down" in the environment.

Glass bottle 1 million years
Monofilament fishing line: 600 years
Plastic beverage bottles: 450 years
Disposable diapers: 450 years
Aluminum can: 80-200 years
Boot sole: 50-80 years
Styrofoam cup: 50 years
Tin can: 50 years
Leather: 50 years
Nylon fabric: 30-40 years
Plastic film canister: 20-30 years
Plastic bag: 10-20 years (???)
Cigarette filter: 1-5 years
Wool sock: 1-5 years
Plywood: 1-3 years
Waxed milk carton: 3 months
Apple core: 2 months
Newspaper: 6 weeks
Orange or banana peel : 2-5 weeks
Paper towel: 2-4 weeks

The above information was taken from the Pocket Guide to Marine Debris from Ocean Conservancy. It's sources were the U.S. National Park Service; Mote Marine Lab, Sarasota, FL and “Garbage In, Garbage Out,” Audubon magazine, Sept/Oct 1998.

Judging by the figures, I'd hazard a guess these would apply when the item is exposed to sunlight and air. Stick some of those items into landfill and in the absence of light and oxygen, chances are they won't break down for many generations. Even newspapers dumped in landfill have been known to be still readable after many years.
 
For disposable shopping bags, I've seen figures anywhere from 500 - 1000 years (but there's many different types of plastics) and cigarette butts up to 12 years. While it may seem odd for leather to take so long to break down, many leather products are treated with all sorts of nasty preservatives to extend their life. The figure for a glass bottle is incredibly long, but at least that's an easily recyclable product. I have picked up old beer bottles on my bush block that are over 50 years old but look as though they were left there yesterday.
 
There's a lot of ifs, buts and maybes in the above list, so if you have stumbled across a comprehensive list or study of  decomposition statistics of various forms of waste with detailed annotations about the conditions in relation to the timeframe, please let me know!