My redecorating rule is to wait till I can't stand to look at it any more. Except in my UK living room where the rule is to wait until I figure out where to stash the contents of the shelves that I was silly enough to fasten to the walls when I made them. I've got a lot of shelves like that, and the rest of the house is full of stuff.
I did redecorate the box room a few years ago, mostly because the hot water tank had sprung a leak and made a big hole in the ceiling. Nobody ever went in there but it just made me feel I was descending into squalour, and that would have messed with my self-image as a fine old English gentleman.
I also redecorated the room I once moved into in a hippie shared house, but that was easy because I hadn't moved my stuff into it yet. When I'd finished, everybody said that it didn't fit in with the scruffy values of the rest of the house. Too mainstream I guess.
The environment inspector said it was "very nice" though. Story of my life, too conventional to be a hippie, too hippie to be conventional.