I occassionally buy something (or a few somethings) used as a "home audition", getting rid of whatever doesn't do the trick for me very soon for little or no $$$ loss. Obviously, many of the participants on this forum do the same. I don't really consider this as a component that I "kept" for only a couple weeks, since it's a bargain-hunters ethical way of auditioning components without leveraging the local dealer.
Of the components I "settled" on, I usually keep them for 1-3 years. If I wasn't married, or if speakers were less easy to notice, I'd try a bunch of different speakers, "keeping" many of them for less than a year, at least until I'd tried a bunch of different "flavors". -Kirk