In the past, I've always subscribed to the 'arc' method of chain wear. When the chain, when off the bike when held sideways, exceeds a 8-10" arc over it's length, it is time for replacement. The chain is held somewhere near the middle and the 'droop' is measured... I have well over 40k miles on my vintage Fuji...

That said, I've worn cheap chains out in a few thousand miles of riding in nasty wet weather, and yet others have lasted 5k miles or more. FWIW, my current chain has over 8k miles on it, and is probably due for replacement, but I'm not looking forward to replacing the 'dinosaur' (20+-yr-old) 6-spd Shimano Ultraglide chain... because I'm an old fossil myself!