The instrument I've had the longest is (I think) my Eko Ranger 6, bought new in the early 80s. I'm not entirely sure as I also bought a Roland D10 about that time that I also still have, but this gets used regularly for practice for open mics with Mrs Zero and for songwriting. String ends flying wild and free to make some here get all twitchy.
The bass I've owned the longest is a Warwick JD Thumb, bought new in early 1988. I'd gone into Musical Exchanges in Birmingham to try out an octave pedal and they handed me another Warwick, a Streamer IIRC, to try it out. It was rather badly set up so I put it back on the wall and took down this one, and from the first moment I played it I wanted it. Absolute dream of a neck - at the time I had a Fender P and a Hohner B2 and it surpassed both of them by miles, so despite the £950 price tag (which Dave the Bass kindly reduced to £900) I took out a loan for £500 and PXed the Fender and the Hohner to get it.
It appeared live on TV when we played in the late early hours on Telethon '90. It still gets occasional outings, and was going to feature in the Marillion tribute that I got dropped from, although since about 2007 I've been 5-string.