I changed my first chain out at 1800 miles, and I think that was about right.
The last one stayed on for 2800 miles, the last 600 or so in very dirty, sandy, wet conditions. It was way too long. I think in the future I'm just going to change at 2000 miles.
Slvoid: Harbor Freight has a 12" digital caliper for $35 for the rest of us :-)