There are loads of ratings and tests for how long DSLR bodies last, but I have never really seen much that describes the expected lifespan of lenses.
Recently, one of my cheap-o lenses (Nikon 18-55mm, ~10,000 exposures) started making some funny noises and the focus ring jams up occasionally. It has been through a lot (got frozen in China, and bounced along trails on the back of a motorcycle in the Rockies), but I expected it to last a little longer than a couple years, especially next to my 20 year old 80-200mm which gets twice the usage.
Anyways, how long it is reasonable to expect a lens to last before needing to be serviced or replaced? Are there any brands/lines of lenses that do better than others (and vice versa)?