The longest-lived fort that I know of is Flarechannel at 270 years.
Now, on an open fort, about the only way to achieve indefinite survival is with liberal use of overpowered traps. Unless you manage to kill off all the goblins in existence, you'll eventually reach a point where the losses due to sieges (in an untrapped or undertrapped fort) begin to pass up the gains from childbirth and immigration.
There are several forts with open designs, including mine, which are decades old. I'm at 50 years at the moment. While I close off certain exits during sieges and close everything up while collecting the spoils of war, I have enough legendaries that sieges have minimal casualties, including a lever mishap where all twelve entrances into my fortress were left open by accident and enemies poured in from all sides (oops).
The following Legends report was taken when the fort was around 38 years old. Unfortunately, I can't take a more recent one as Legends Viewer stopped working when I ported over to LNP r18
. More soldier have died from old age than from battle.
I also don't use danger rooms. I included a hammerdwarf and an axedwarf in my embark team and had them training from the start. Until I started getting legendaries, I kept squads small, usually 3 with 2-on/1-off scheduling. I also don't have native iron on my map, but I do have flux. I quickly traded my way to steel weapons and iron armor by the time the first siege arrived, and steel everything soon after.
For Forgotten Beasts and Titans, I use cave-ins. I don't use weapon traps, and I only have 1 barely used corridor with cage traps for catching things like dragons.
The second half of my fort's life has actually been a lot easier than the first. Once you get several legendaries in the military, training speeds bumps up significantly. At any given time, I have 32-36 dwarves on active duty and 10-12 on reserve (newbs in training). By the time a spot in an active duty squad opens up, there's a recruit with 10+ skills ready to join
.