My longest, which I unfortunately don't remember the name of, lasted around 25 years before I got bored and quit. It had canals to channel magma anywhere I wanted... A huge Bloodbowl arena in the sky (which I liked to drop trolls from)... towers, skybridges, a 50+ z-level magma pump stack, huge magma resivoirs, a giant courtyard... It was pretty big. And not defensive at all. I kept it safe with masses of archers, rather than traps. Oh, and magma. I had the outer courtyard rigged so I could fill it to the brim, if I wanted to, and the inner courtyard too, like a giant bowl of magma. I never did, though... figured it would be too much to clean up (plus, the lag was unbearable). I only ever flooded the outer courtyard.
Did I mention it was in a terrifying biome? constant zombies. It was fun... I kind of miss it.
The longest I've ever heard of was 200+ years... I don't remember its name, either, but from it came the fable of Catten and the Giant Eagle. The fortress was huge, armies broke upon it like waves upon a cliff, there were dead dwarves all over the place, embedded in the constructions (which were sky-high), it had a carefully-designed temple that stood hundreds of feet tall, it had sky apartments, workshops on piers on an underground lake fed by a waterfall... It was incredible. Made anything I've ever done look like a toy.
edit: I was ninja'd by six responses... wow.