I have one of these too. Measuring the chain wear by the ruler approach was always a pain for me because it is a pretty small amount and I was never confident that I was getting a good reading. The Park chain checker measures the gap. If the checker falls into the gap, the chain is worn. If it does not fall into the gap, it is still good. It is quick and easy and virtually fool proof. It also has 2 measures that you can use, a .7% wear and a 1% wear. I use the .7% so if it hits that, I can go get a new chain before I start eating into the rear cluster.

The rate of chain wear is determined largely by how clean you keep your drive train. I read a post from a guy who cleaned his whole drive train after each and every ride. He said his chains lasted for over 10,000 miles. I clean my drive train about every 400 miles and/or everytime it gets wet.