I guess the longest was a Cold Steel Voyager clip point -- 1998 through this year. There was a Vic SAK, the Spartan I think, that I carried through the 70s and early 80s. Then I got a Buck 110 that I carried for about 8 years in the 80s. Then a SAK Executive through most of the 90s until I got the Voyager. I tend to get a knife and stick with it.
Oh, and I've been carrying a SAK Champ on my belt for several years. Rarely use the blade, as my other EDC covers that duty. But the many tools come in handy all the time.