I've had 4 60 gallon water barrels on my back porch (exposed to the elements) for 8-10 years now. They still hold water and are in decent condition. They are covered and not exposed to the sun however, I suspect the sun would deteriorate them. I keep recycled milk jugs in the basement, which I have heard deteriorate (as a previous poster mentioned). Typically my milk jugs are used for camping (freeze them for my ice chests), so they are replaced frequently.

Cheers,
Tim