I usually get bored with a fort before it dies of FPS (20 years in?) but then I limit my population in the init file, and slaughter any animals I don't need.
That said, in a volcano recently I had my FPS reach 11, and decided I probably should leave the fortress and start again. Instead, I left the game to run in the background for a bit, returning to unpause for moody dwarves, basically trying to give the fort a 'year off', leaving all instructions in the manager screen turned off. The dorfs hauled, partied, prayed, hauled, sold all the junk of the fortress to the Humans, hauled, drank, ate, pulled levers, and hauled. After about a year and a half, the fort was back up to 75 FPS, and I kept it going after that for a few more years, before having an unfortunate incident with a dusting Forgotten Beast and finally moving on. At the time, I thought it was a problem with nothing being moved around because everyone was so busy, but later I found a set of locked doors, so it's possible all my FPS issues were a cat chasing a moth down a corridor and failing to make it through the door. The return to a reasonable FPS was gradual though, and we dumped a lot of objects and sold a lot as well, so it's possible I was also just getting rid of enough stuff to let the FPS raise that way. Who knows?