I threw out a duvet yesterday that's been on my bed (either on top to keep me warm or underneath the bottom sheet to provide extra padding) almost every day since I bought it. It was a super-lightweight one that never had a cover on it and just got washed as if it was a sheet. It was getting a bit tatty, with some of the stuffing coming out of it but I'd probably have still kept it for a few years longer if the cat hadn't thrown up on it.
However I reckon that I probably got reasonable value out of it as I bought it in 1975 ;-)