Just wondering, about how long do your forts last? I'm ashamed to say my longest lasting was 5 game years, as the goblins got in through an unfinished surface construction...
forts normally last indefinately for me, until the fps gets horrible or i get bored. normally the first. i normally try to make my forts in the same world, so i have a few if i want to try adventure mode.
in 40d once i reach a certain "critical mass" my fort tends to be undestroyable. my first huge fort had several near show stoppers once i got past that critical mass, which was accidentally digging into a high pressure water pipe with a massive resovoir (good drainage planning ment it could all flow through without drowning too many dwarves) second near fort end was accidentally causing a cavein down my chasm, taking out a half dozen bridges packed with dwarves that were carting ore back and forth. several much loved dwarves died, which was tough.
final near showstopper was taking on the hfs. all my noble tombs were in the adamantium veins, and were very full of items and remains. the hfs smashed all those up and half my fort went to go pick up the liberated items. in adition all the tame cats were in the area hunting spiders, so their loss along with literally half of a 130 dwarf population also caused tremendous unhappiness and tantrums, but was still surviveable. without the excitement i would probably have gotten bored of playing for so long.
im yet to find out the survivability time for the new version, though. tougher goblins, titans and the underground are all new. i havent played nearly as much of it though because the slowness on my old computer is nearly unbearable, and breaching the new HFS brings things pretty much to a halt.