It came as a big surprise to me the first time I found out my front chain rings and rear cassette needed replacing. I used to think they were steel and bike riding surely didn’t put much preasure on them so they should last a long time right? I was wrong
Since the chain is exposed to the air and road dirt and dust, small particles of grit get in side the chain and wear it away. A chain inside a sealed chain case would last much longer but we couldn’t change gears using a derailleur with a chain case. You can extend the life of a chain by keeping it cleaned regularly and well lubricated. If you ride where there is less dirt and dust it can also last longer. Depending on how you care for your chain and the environment you ride in you can get anywhere from less than 1,000 km to over 10,000 km out of a chain. 3,000 km is often quoted as a typical life for a chain on a derailleur equipped bike. How far is your commute?
As the chain gradually wares the distance between the links gets longer. As it does it means it doesn’t mesh perfectly with the teeth on the front chain ring and rear cassette. Instead of the chain distributing the chain tension across a number of teeth it is concentrated on just one at a time. This causes the teeth to ware to match the increased distance between the chain links and the chain tension becomes more evenly distributed again. Over time the chain and teeth ware together.
If you leave it too long and just replace the chain the teeth on the front chain rings and rear cassette may have warn too much for them to mesh with the chain and you will find your chain skips. So you will need to replace the chain, the front chain rings and the rear cassettes.
If you keep a close eye on how much your chain is waring you can just replace the chain before it wares too much without the need to replace the front chain rings or rear cassette. If you are careful you may get through three chains before the front chain ring and / or rear cassette have warn too much and need replacing.
You can buy special tools to measure chain ware. Here is a link describing how to measure chain ware with only a tape measure (the technique is at the bottom of the page).http://sheldonbrown.com/chains.html