I bought a new road bike with Campag Athena 11 speed in March last year. To the end of Jan it had covered 9000km.
In all that time I hadn't changed the chain because I don't have the Campag tool, and the KMC chain was not available. I finally got a new chain recently - unsurprisingly the old one was very stretched - so I bought a new cassette as well.
The thing is, the new chain on the old cassette runs very smoothly, and even dismantling them and comparing the cogs I can't see any change in shape or size! I don't know what Campag are making them out of, but it's fantastic stuff.
Everyone I've spoken to is convinced that after 9000km on one chain the cassette will be useless, but the prevailing advice is also to change the cassette when there are issues, and there just don't seem to be any.