My two cents...
As Alan mentioned, some manufacturers recommend replacing a chain every 1,500 to 2,000 miles. (Campy makes exactly this recommendation for it's 11-speed chains.)
I disagree with that recommendation because it's based on the following assumptions:
1. All cyclists have very similar riding behavior and therefore wear chains equally (poor assumption).
2. All cyclists ride under very similar weather conditions all the time (poor assumption).
3. All cyclists don't maintain their chains very well (pretty good assumption, as most don't).
4. All cyclists don't know how to determine if their chain has too much wear (excellent assumption).
I'll advocate that a chain should be retired when it's reliably determined that it has stretched 0.5% (approximately one sixteenth of an inch) over 12 inches. If it has 500 miles on it, and is stretched that much, it should be replaced. If it has 12,000 miles on it and has just reached one-sixteenth inch of stretch, it should be replaced.
Why replace a chain at 2,000 miles when it still has plenty of stretch left in it? I suppose if you're a chain manufacturer, you'd be happy to see people doing that. Personally, I'd rather change my chain when it's worn out (like my last one was when it reached 0.5% stretch at 5,235 miles), and save on a couple of unnecessary chain replacements (at $60/chain = $120, not to mention the savings of prolonging the cogs and rings).
If you keep your chain clean & well-lubed, you can go a lot farther than 2,000 miles without having to replace it. I don't care what the manufacturer says about replacing it at a set number of miles (ignoring how you ride or how well you maintain or don't maintain your drivetrain, or whether you're riding a lot in mud and grit, or only on clear, sunny days).
If you have a way to accurately measure the wear in your chain, you can avoid replacing chains that have lots of good miles left in them (and save that chain money for the eventual replacement of your cogset and chainrings).
Keep it clean, monitor the wear, save money, (and enjoy a properly functioning drive train). If you don't want to bother with it, who cares? It's your bike.
One more point: determining the wear on the cogs and rings has not been addressed. That's for another time. So are tips on how to maintain a chain and measure its wear. I'm sure Feist, our master mechanic, can come up with good suggestions.
Pasta lumbago!
Greg
References:
More on how chains wear out and what chain checking tools on the market (in 2009/10) are accurate:
How the 1% stretch number is determined...
All the chains on modern road bikes have 24 links per 12 inches (12 with outer plates, 12 with inner plates). This is a global cycling industry standard. From a fixed point on one link of a new chain, to the exact same point 24 links away from it, it's exactly 12 inches.
1% of 12 inches = 0.120 inches
one-eighth of 1 inch = 0.125 inches, so you can appreciate that one-eighth of an inch is really, really close to 1% of 12 inches.
If a chain has worn and you measure from the same fixed point to the same fixed point 24 links away, and that distance is 12 and one-eighth inch, then you know you have 1% wear on the chain.
With today's tighter tolerances for chains (to work well with 10 and 11-speed cogsets), it's been found that 1% of chain stretch is too much. That's why the recommendation is to change out the chain when it reaches 0.5% (or one-sixteenth inch). As noted above, the number of miles on the chain doesn't really tell you how much stretch is actually in the chain. You have to accurately measure it to really know.
