I've gone for seven years on sudden death difficulty (vanilla sudden death, no adjustments to anything) and just reached seven years.
To add to the challenge, I made some additional rules:
No traps involving hatches or mechanical walls (these are too powerful)
No torches (my gnomes aren't scared of the dark, after all what's the point of all those underground enemies if you never fight them)
No use of 'dig ramp down' or similar to discover mineral veins (that seems like cheating)
I did not build a wall around my kingdom, or wall in. (I did this so I could spot bears more easily, though I don't need to kill bears anymore)
No big stone stockpiles or selling raw stone to the merchant (I did eventually build a small one just to tidy the place up)
There was one strange thing about my game, I never met a single beetle. I did plan for them and was disappointed when they didn't turn up as they are great for training my squads.
Here's a link to the savegame:https://www.dropbox.com/s/t9gw5k490bkwuyx/world01.sav?dl=0
It's still going strong, sometime I want to make a secure workshop area and get a single non combat automaton in there who never sleeps, because all my best civilian gnomes are now dead. One of the original 9 gnomes is still alive, so he is now the king
So if anyone else wants to try seven years of sudden death, know that it is possible.