I seem to be using a 100 year rule.
Me too. Earlier this year I sold off most of my dead tree Traveller collection, and replaced it with 5 CDs containing all the canon and magazines. This despite the fact that I have played Traveller exactly once, and that was in 1983. I don't look at it as a game, but as a world in which I can create stories and let my mind wander as it will. I have many novels on my shelf that have not been read in 2-3 years, and some haven't been touched in 5-10. I see no reason to sell them though, as I know I'll read them again someday.
Harn is the same thing. I've never used HarnMaster, and it's unlikely I ever will. The few times I've been the GM, the scenarios were supposedly in Harn, but they were so generic it could have been the Forgotten Realms and nothing would have changed. It's been at least 7 years since I did any gaming at all, and over 18 years since I did any real role-playing. I'm a storyteller now, not a player or GM. Harn is the setting for most of my fantasy stories, and my mind regularly wanders around it. I like having it around for this reason. I might not touch a book for a year or three, but having it around as a reference is a very good thing.