I just changed my chain at 52,000 miles, I bought the bike at 25,000 and it had a newish looking DID chain on it then so not too bad.
When I changed the chain I put the uprated sprocket nut on so it had been running on that for the whole 52K Got a Regina HD chain and sprok set for £130 at Essential Rubber near the Blackwall Tunnell, good value I thought!