If you clean and maintain your bikes cogs regularly you can keep the wear on them down and keep them from needing to be replaced for thousands of miles.
The first thing to do is to find out how many cogs you have. The newest generation of road bikes probably has a nine or ten cogs in the cassette. This is only a recent development since for the longest time only seven or eight cogs were available on a cassette. This matters to you because a ten speed cassette only works with a ten speed shifter and dérailleur while a nine speed only works with nine speed parts. If your parts are from Shimano or SRAM they are more or less interchangeable and work on shimano hubs (which is the majority of the market). Much more rare are campagnolo hubs which only work with campagnolo parts. On a cassette the smallest ring will have somewhere around 10-12 teeth while the largest will have somewhere in between 24 and 34 teeth.
On your bike as you ride it, your drive train parts will need replacing. Probably the single most expensive drive train part to replace is the cassette which can run you as little as 50 dollars to up to 250 dollars. If you keep the cassette in good condition though, it can be one of the longest lasting parts on your bike! To keep your cassette in great condition though, you need to do two things. The first thing to do is keep replacing the chain on your bicycle when it gets worn. A worn chain wears out your cassette.
If you keep your sprocket clean in keeps grit that grinds itself in and lies down the teeth the cassette will also be much cleaner. To do this you will need to take off the rear wheel, and spray it with a citrus based cleaner. Then use a toothbrush and scrub that cassette-HARD. You need to get in between cogs and into those tough to reach spots. One way to get deep into there is to get a rag and sort of floss between the two cogs. By doing these two steps you can get a cassette to last 10,000 miles. If you do not do these things a cassette is only going to last a couple thousand miles before you need to make a costly repair.
Source by Jeff Hendrix