What does it take to wear out a bike chain?

My road bike is equipped with a Shimano 9 speed chain which is as old as the bike (6 years old) and despite a fair bit of use over the years it remains in good condition.

Despite a slow start to summer, I've been determined to increase my mileage this year.  I've already logged more miles than last year, so maybe this will be the year my chain finally wears out.