It all depends on the car or truck, and how much I like it...or dislike it. Also depends on how much I have to sink into it to keep it on the road. I like to shoot for 8-10 years...or more. And have succeeded a couple of times.
I totaled one that was only 3 years old, but I would have kept 'forever' otherwise. I had one, a '77 Camaro, that I traded in after only 4 months.....before I could kill myself in it (fast cars were meant to go fast, right). One night I was coming back from out of town, cruising at ~125mph, and got passed by a big ol' Buick Electra like I was standing still. Next morning I was at the dealer's hammering out a deal on a new van to customize (which turned out to be the afore mentioned one that I totaled).
I had a GMC pickup that I really liked, but unfortunately I needed more passenger room, so I traded it in after a little less than 2 years on a GMC van. A friend of friend bought the pickup about a week after I traded it in. I had had absolutely
NO problems with it whatsoever. About 2 months after the guy bought it, it literally spent more time in the dealer's repair shop than on the road. It started having electrical problems, and after replacing nearly the entire electrical system....including the wiring harness twice.....GM declared it a lemon, and the guy got a different truck. When I first traded it in, I was kind of miffed at myself for doing it, but after seeing what happen to it 2 months down the line.....I easily forgave myself!
My Dad was always one to drive it 2 years, and trade it in on a new one. Rinse...repeat. After he passed away, my Mom said she thought it was time to trade her car in since it was now 2 years old. I convinced her to drive it a little longer. It finally started becoming less reliable, so she finally traded it in on a new one when it was about 8 years old. Her new one is now 4 years old, and she plans to drive it 'til the wheels fall off.