Well, if we're talking serious forts here, then usually around 7 years.
I did once manage a game that got up to 35 years though, with immigration turned off. I was just starting to get grandchildren old enough to start working when an unfortunate sequence of events involving a lever, a wild turkey, a skittish dwarf, and a goblin ambush resulted in unavoidable catastrophe, concluding in the deaths (by attackers and later madness) of all but my baroness and my militia captain.
It was a sad abandon of that fort.
I did however come back with an adventurer (actually, one who I had played on before founding the fort) and recruited some antwomen I found living in the caverns below the fortress.
My memory of that place is still... emotional. I had even thought of writing a story of events in my fortress, and had carefully catologued births and deaths and the names of nobles over the years. Took lots of screenshots too. It's strange to get attached to a group of pixels, but really this is the only game I know of where, if you look close enough, you really can get attached that way.
Anyway, that was slightly off-topic, but I am hoping to get something that sustainable running again once the next release gets stable. I'll probably pepper the dwarven "sites" with adventurers as well, and hope for an epic tale. The best forts are the long forts.