I used an old AT case (I think it started out holding a 286 or 386) with a P II level Celeron (333MHz) until 2006. The case must have been almost 20 years old.
It's been replaced by an original Cooler Master Stacker in 2006, which I will probably keep in server duty for a few more years. Thanks to the flexible 5.25 front, I can eventually put hot-swap front bays in, and really make that case the next best thing to a rack-mount storge server. It's been through it's first complete fan change two years ago though, which has helped make it living-room friendly, at least from the sound levels.
In the same year, I got a CM Elite something or other - basically a slightly-better-than-throwaway case - which still houses an E6600/8GB/GTX260-216/Early Samsung SSD for my mother. Fans will have to be replaced eventually.
My lst build, around 2.5 years ago was around a Fortress FT03, right when that case came out, and I'm pretty sure that case will be in use for a while yet. Projected upgrade for the internals is another 2 years off, they've just gone through the midlife RAM+GPU upgrade and then we'll see whether I'll give it away to a loving home, or just put in some new hardware.
On the other hand, I'm usually recommending people, who ask for builds, to get new cases. Why? Their cases are usually five to ten year-old throw-aways, made entirely from steel, lacking USB 3.0, with the worst front audio imaginable, and generally so bad, that when I show them a 60-80 euro Lian Li, they are just awed, at what an aluminium front can do to a PC's looks.
Especially, since lately I've been building a bunch of micro-ATX/mini-ITX builds, or used those as starting points, there are plenty of nice looking cases, that aren't too expensive. Anyone having a good case, probably knows enough about computers not to bother me about them.