Twice a year I change duvets, 4.5 tog for Summer and 10.5 for Winter. We have one double and two king size beds in total. At this time of year I wash the lighter ones before storing but I buy inexpensive 10.5s and bin them rather than try to wash and dry them as I believe it would probably cost more.
DH doesn't think they need to be washed or binned for several years (MIL never did). I think that all the sweat, drool and other bodily fluids makes them rank. I also wash the feather pillows one a year (summer) and change mattress covers and pillow protectors four times a year and he doesn't see the need to wash the pillows - ever!