Chains wear out. When they do wear out, they also start taking cassettes and chainrings with them. Just last week, I replaced a chain on a bike that had also killed the cassette cogs and all three chainrings. Not a cheap repair. It all could have been avoided by replacing the chain.
The question is "how do I know when to replace my chain?" If you know from historical data that you get X,XXX miles out of your chains, replace it after that many miles. In the shop, I use Shimano's TL-CN42 chain wear tool. It's more accurate than the simple tools that push against two rollers and give you a percentage wear. Some of these tools will show that a brand new chain is almost at the recommended replacement mark of .75% "stretched."
However, you don't need a fancy tool to measure your chain. A simple metal ruler works great. Each link in a chain is 1/2" from pin center to pin center. Twelve inches of chain will contain 24 links. A brand new chain will measure 24 links at 12" on center. If you measure your chain and find that 24 links is measuring 12 1/16", it's probably okay - for a little while. The old train of though was that to replace the chain at 12 1/8", but with modern drivetrains requiring a higher tolerance of adjustment, replacing the chain before it gets to the 12 1/8" mark is prudent.
I didn't start out wanting to write about chain wear. I came here to talk about shift cables. Wait? Shift cables and chain wear? Ah, yes, they go hand in hand. I'm talking road bikes here, and specifically road bikes with Shimano shifters, and even more specifically, Shimano shifters with external shift housing from the 7800, 6600, 5600 era and prior. Although, the recommendation is wise for any shift/brake lever.
A brake lever body isn't very big. When it also houses the shift mechanism that is essentially a spool that takes up cable pull, it all gets pretty small. It's not uncommon for shift cables to break inside the shifter mechanism. When they do break, they can be a horrendous pain to extract. Frayed broken cables do their best to make removal difficult. But wait! There's a way you can avoid the hassle of surgically removing broken shift cables from your shifter (or having me do it in the middle of your ride while you wait). When you replace your chain, replace your shift cable as a general, routine service. It's highly unlikely you will ever get stuck out on a ride if you stick to this routine.
Here's a shift cable I replaced a couple of days ago on a bike that came in for service. The chain was just at the point of needing replacement. New chain and at Black Mountain Cycles, that means a new shift cable as well. Good timing because the shift cable was just starting to break.
Go measure your chain. Replace it if it's worn and do your shifter a favor and replace the cable as well.
(What's playing: Johnny Cash On The Evening Train)