It doesn't really stretch, the internal rollers wear out over time. I don't know why anyone has to replace a chain in 1500 miles. I lube and wipe down my chain (D-A or Ultegra usually) at least every 100 miles, which means 2x a week. I typically get 3,500-4,000 miles on a chain, replace the cogset every second chain. Sure I use the Park tool to measure, but you can tell by shift degradation when the chain is starting to go.