I found a good site on this subject from:

Indoor Environment Program
Energy & Environment Division
Lawrence Berkeley National Laboratory

http://www.lbl.gov/Education/ELSI/Frames/Sustain21-f.html

All light bulbs are sold with a life expectation. Why would a UV light be any different.

The spectral output of the lamp changes with age. For sterilization, there is a specific wavelength of UV which is most effective (I don't remember the wavelength off hand). One can expect about a year of effective sterilization with the typical bulb.

Quite. It is good policy to change the lamp at the manufacturer's recommended intervals, so as to ensure system performance.

probably UV-B 320-280nm. UV-C 280-100nm is too dangerous, UV-A 395-320 probably wouldn't give you as complete a bacterial genocide quiickly enough, and what cwarner7_11 said about the spectrum changing with age is correct.

You and cwarner are exactly right. I use the 280-100nm type in an array to sterilize maple syrup and according to specs must replace after 2000 hrs of usage at which point they go into my greenhouse. Exposure to this frequency is quite dangerous and can leave severe burns.

What causes the spectrum to change

There are many things that reduce the life of a UV lamp but the more important reduction of sterilisation capacity comes from the dirts accumulating on the quartz tube isolating the lamp from the water. In professional systems, some automatic wipers are used to clean this tube. A sensor is also used to monitor the UV intensity through the water. It is important since the turbidity of surface water changes on a daily basis.

As for the lamp, you get mechanical failures from thermal stress and mis-handling. You also get metallisation inside the glass from the heated electrodes. In high capacity lamps, the glass can also be "frosted" by the UV intensity. Finally, the heated electrodes can eventually fail.

That covers the bulk of it.

For domestic systems, a wipe of the outer glass lamp housing (not the lamp itself, which ideally should not be fingered), with a new, plastic-based, abrasive cloth and some vinegar works wonders!