Now I'm laughing at myself.

I have no idea how many miles I get from a chain. I'm constantly fiddling with my bikes changing cranksets, cassettes and the like. I'll leave a used chain on if I install a used cassette but I always install a brand new chain with a brand new cassette. For maintenance I ride until the chain starts making noise, then I'll clean it with a rag and MAYBE some WD 40 and dribble on the least amount of chain lube that I can manage.

If I suspect chain wear I'll measure my chain with a steel ruler (don't trust chain checkers) but I can't remember the last time that I changed a chain just because it had elongated.