My last car, "MARSRVR" is a red 1996 Suzuki Sidekick JX 2DR convertible 4x4. I bought that new too, my first bouncy kid car straight out of college (it's also nicknamed "Tigger"). 210,000+ miles on it and its still going strong. I still have it, always will. Now that its retired from commuter beater duty I'm thinking "lift kit".
I only buy a car every 15 years when the auto industry pulls its head out of its collective ...um..."the grass" and makes something worthwhile. I'm sorely disappointed with the long runs of look-alike drive-alike "slugmobiles" in between. I'm also sorely disjointed with the American auto industry. I wanted to buy American this go-around, (my Suzuki's actually Canadian) but Detroit can't manage to fit an engine in anything smaller than a Camry without exploding.
SABQT is part of my stable now, will be forever, just like MARSRVR. 15 years from now I'll be 55 and looking for a luxury wagon to haul my old bones around gently in style. I'll probably go steam-powered hybrid next and burn junk mail for free.
I keep cars forever. I just add to my stable as different needs arise.
For me I just usually wait until I fall out of love with it. When I dont feel like driving it anymore or I just get bored of it. I only buy cars that I enjoy owning but when I dont enjoy it anymore thats a pretty clear sign for me to move on and get something new.