I bought a new vacuum the other day and discovered that the belts on vacuums should be changed at regular intervals.  I never knew that needed to happen.  The same is true for bicycle chains.  If a bike is ridden regularly you can expect a chain to last anywhere from 500 miles to 2500 miles depending on the rider and the riding conditions.  The harsher the conditions - like Missoula in the winter, or the heavier the rider the shorter the lifetime of the chain. 

Checking chain wear is very easy and quick.  You can stop by the shop and have one of the folks here place the chain checker on your chain and they will tell you how your chain is doing.  We also sell chain checkers here at the shop so you can have one of your own.  Keep in mind that if your chain is well worn you may also have to replace your rear gear cluster.