Rear derailleur lifespans How should I know when my rear derailleur is getting worn? I can measure/inspect chains, tyres and brakes;  gears tend to show problems on a new chain if they're worn. But when changing my chain at the weekend I noticed the that there seemed to be quite a bit of play in the pivots - more than I'd expect from mechanical bits in general. Also the jockey wheels were quite worn (and didn't run very smoothly even after a good clean and dry PTFE). 

In my specific case it's only Altus, with about 20 000 km on it, and the bike doesn't get cleaned as often as it should.
I cannot comment on derailleurs in general, just my own experience. 

My retro ride is 35 years old, and in that time I've ridden it about 150,000 km. It has Campagnolo Record derailleurs, and both are fine. Shifting is still good with the downtube shifters. The parallelogram is rock solid. 

I cannot claim to have maintained it especially well. It's only had about 10 chains and clusters in that time. 

Observing the milage of others, I think the main end of life event is a crash. Some people do seem to wear the jockey wheels, perhaps due to running lower quality chains. 

My conclusion is that it all depends on the initial quality. We get what we pay for.