as long as you are happy seems like the best answer.
I posed a similar
question a while back, and got the answers I pretty much expected: they seem to degrade verrrrry slowly, such that you might not notice; as long as they tune easily, and sound good to you, then there's not much point in changing them.
[edited: thanks for the comments guys, sorry I didn't respond back then.]
I decided to try a new set anyway (after two years, say an average of 5 hours a week, and my hands don't sweat much). Again, it's hard to be precise because they take a few hours of playing to settle in, but the new ones were a little better.
To say they were brighter, or that the old ones were anywhere close to dead, would be grossly overstating it. I would say I heard a bit more harmonic complexity and richness, a little more character or liveliness. However, this was in a hi fi setting - alone in my living room, with a good rig and great acoustics. I suspect that in a live performing situation, it would have been difficult or impossible to tell, and the difference might easily be overwhelmed by even the smallest control changes you might make to get a bit more presence, or even just switching instrument cables.
Even if I start playing more frequently, I expect it will be another 2-3 years before I bother replacing them again, more because it will just seem like it's about time than because they've hit the end of their useful life. At this rate, cost isn't a factor at all.
(Message edited by bob on April 17, 2006)