I look to replace chains at about 1-2k miles in winter, or 2k+ in the summer. Ymmv.

Honestly, unless you're on a 11sp, chains are cheap compared to any of the other parts.

When I was cycling 3000km/year, I replaced the chain every couple of years.

But it seemed to depend massively on how careful I was with cleaning, and where I was cycling. 99% on roads, with regular lubrication, and the chain lasted ages. Then I moved house, and cycled the same distance, but on gritted paths, and more often in the rain. Chains could be ruined very quickly if I was lazy, and didn't clean off the mud and grit.

2 years times 900 km sounds like a short-ish but reasonable life for chains.

OK. Guess thats how it is. Thank you all.

600 miles a year is relatively low. Most people replace chains around 1500-2000 miles but it can really vary.