I replace chains every 6500-ish km and they have more life in them at that point, but I have an important race coming up or something and want everything to be in perfect condition, or last time I changed my chain I installed an OSPW and hence had to put a longer chain in and so on.
Cleaning and lubing with quality stuff, and the chains are Dura-ace 11 speed. It absolutely matters - not all lubes are the same and chain quality differs.
Since they are in reasonably good condition still, I didn't toss the last couple or so in the garbage. Might use them for kids' bikes or something.