I'm confronted with this question often; but my answer is hopefully never. When that day comes, I'm no longer interested in audio. But that doesn't mean I run out and buy more or different stuff. I tweak my room, move my speakers, reconfigure my racks, reorganize my records and CDs, check that my stuff has the latest firmware, and on and on.
To me the fun is in engaging with the environment and the equipment. May I never be fully satisfied.