For me it is when I start feeling overwhelmed, when I no longer feel joy when I see it, when I no longer play with it, can't find anything, spend money I do not have, when I can't work in my studio because of all the stuff. I agree and think it is different for everyone. Two shelf units would be oodles of fun for me but your friends house would scare me. I do not think it has anything to do with if you ever use it or not.