I am still flying three planes I built up back circa 1990. One plane a 40size pattern plane that has wore out a Saito 65 after a few years andis now sporting a Saito 80. The paint is getting a lot of cracks init, but the plane still flies great.
The other is a cheap Tower Starfire 40 that wore out a couple of 40engines (running a tuned pipe didn't help engine life). The plasticbits and pieces on the Starfire is starting to crack and break off, butit still works. I was using it to test new RC systems and receivers for a while. As if the radio flakes out on me it won't be a big deal.
The third plane is a Airmadillo Combat Twin 40. Basically it is more of less indestructable. Great for those crowded busy flying days at the club field. You don't want to mid air a Airmadillo plane. I did crash it a while back when one of the nylon wing bolts broke off and it rendered the ailerons pretty much useless. But a little work with a hammer and a couple of crescent wrenches and a pair of vice grips had it all back in shape again.

I also have a Sig Senior Kadet, that I built up back around 1987, butit is semi-retired, until i decide to recover it as the monokote is allraggidy now. it was built as a electric plane first using lots ofnicads and a Astro-Flite 40 geared motor. later I converted it to fueland am running a old OS 60 FS four cycle with the open rocker arms andpushrods. it is fun to just putt around at 1/4 throttle. i diodaccidentally crash it when it was electric when the battery ran out ofpower and I couldn't fit it in between the power lines on the landingapproach. but it was quickly fixed and I converted it to fuel powerthen. I did send the Astro-Flite motor in for repair and it came backthough, I just haven't used it since.

I know some guys retire a plane after a season of flying. I usually go until the plane is deemed unflyable for whatever reason.
I replace the on/off switch from time to time, and the battery before it gets too weak. Plus a servo every now and then too.