Depends very heavily on riding conditions and on terrain. In the sense that some people, for instance those who commute on their gravel bike tend to stay on the same 2-3 rings of the cassette (assuming they don't live in the mountains) which obviously will wear those out while sparing the others.
I ride about 13k kms (about 8.5 k miles I guess) a year and typically go through 3 chains (but I keep 'm waxed) every 18 months and one cassette every two years. But although I ride every day regardless of the weather, 90% of that distance is commuting over decent roads.
That said, even if the cassette is still fine, it's not uncommon to experience some slippage right after putting on a new chain. Usually goes away after a couple of 100 km.
If you go through 3 chains in 6k km, you myust be riding some seriously rough stuff.