I just replaced my chain after 9000 miles. I had measured it a few times and never found it to be stretched (perhaps due to my extremely weak pedaling), so I never replaced it. When I did, I also had the cassette replaced and the LBS misaligned things, resulting in the derailluer breaking off during the second ride. So now have a new chain, cassette, rear derailuer, and I got a new bottom bracket for good measure. It feels like a whole new bike.