The longest you have owned any gear?

I still have a Denon DP 31L. Kenwood M2 and C2 amplifier and Energy Pro 22 speakers.

over 30 years old

Polk Monitor 7B speakers and Optonica RP3636 turntable both purchased new in 1978. Still in rotation in my second system. 

Audio Research SP-7 pre-amp and D100 amp, and Dahlquist DQ20 speakers all bought in 1988 from Wilson Audio in New Orleans, Louisiana. Audio Research gear sold in 2017 (for 47% of original cost). Looking to sell DQ20s.

Revox A77, bought new in '72. ESS TranStatic I's, bought used in '81. Nakamichi BX300, bought new in mid-'80's. Original Quad ESL's, bought used and restored in the early '90's. Townshend Elite Rock turntable, bought new around the same time. An ARC LS-1 and Bedini 25/25, used in mid-'90's. Stax Lambda Pro Earspeakers and McIntosh MR78, used in late '90's. Still own them all.

Polk Model 10A from 1979. They're in my home theater system now. I still use a Burwen Research 1201A for filtering hiss when I dub LP's to disc. A 1977 Army purchase!