LCD monitors can and probably will last you at least a decade if not two if used normally. I haven't read any tests of leaving an LCD on for 24/7 for long periods of times, but unlike CRTs or plasmas they don't suffer from burn-in images. However it's possible for them to acquire stuck or dead pixels. Again I have no idea if prolonged use is the cause of this. Most of the time dead/stuck pixels are defects in the manufacturing process.
It's also hard to gauge if LCD's fade or lose color quality over time, because any monitor old enough for us to test the effects of age on the order of ten years or more were based on older technologies and had worse color reproduction to begin with. I have a 15" Compaq monitor from the old Athlong Tbird era (circa 8~ years ago) and it still works just fine, although the colors are noticeably worse than my Samsung 2493HM and my older Viewsonic 19" (2005ish). The Viewsonic meanwhile looks almost as good as Samsung although it does trade color accuracy for a 2ms response time right out of the box.