How long do starships last?
Let’s take two extreme examples…
A top of the line, no-holds-barred, state-of-the-art starship…
…and…
…a Hariven.
Assume that both are lovingly cared for by their owner/operators and that both are kept stock.
I think finding ones kept stock might be the hardest part of that equation. 
Oh, I’ve got to think there are a few purists in Space Elf Land (but that may be my own prejudice at work). State of the art owners might jump at every possible improvement while Hariven flyers (at least some who fly steel cuz they want to) might be more inclined to fly stock (keepin’ it simple, stupid) than constantly be on the look out for the next bit of AWESOME.
Amusingly to me, by picking the two examples at the extreme ends of the scale, you’ve picked the two examples of Most Likely To Last Forever.
- The former will last forever because it has incredibly sophisticated self-repair, mutual-repair and perpetual-maintenance systems that, given continuing energy input and occasional replacement mass, will keep things (including themselves) working come what may, forever.
- The latter will last forever because it’s a junker that contains very little that can’t be fixed by a gorilla with a sledgehammer, monkey wrench, soldering iron, and duct tape. If you have a broken Hariven and access to a wreckyard, you have a working Hariven. Sure, give it a few years and there may be no original parts left and forty-three systems with warrants out on you for improper radioactive waste disposal, but the important thing about the Ship of Theseus is that it’s still a ship, belike.
(The middle of the range usually keeps going for a few centuries, given proper maintenance, before something really expensive to replace breaks and depreciation makes it more cost-effective to replace than repair.)