Chain checking gauges tend to read high for a number of reasons which I won't bother going into. Measure your chain the reliable old fashioned way, as described by JoeJack above, and replace it if stretched beyond 1/16" over 12". The rate of cassette wear increases as the chain wears, so the per mile cost of running your drive train increases using old chains.

I'm not a fan of using a flat mileage basis, since the chain wear rate varies so much person to person, with some burning out chains in a little over 1,000 miles and others going for 5-10,000 miles. Everyone rides differently, under different conditions, and has different attitudes about chain care, so averages are meaningless to you're own situation.

BTW- follow good chain lube and care practice to maximize your chin life, and that'll save you dough on cassettes also.