If you consult wikipedia, the lifespan of the Siamese fighting fish (or Betta splendens) is somewhere from 2-5 years, with the note that the age of Bettas purchased at pet stores can sometimes be hard to discern. The plumage on the male Betta apparently comes into "full flower" when the fish are about 9 months old. This is when they're the most attractive, and therefore the easiest to sell. So you never really know.
You never really know with relationships either.
Now, I realize that jumping from a sushi-centric story to a story about pet fish might give some readers the creeping horrors, so let me preface the the rest of this tale by mentioning that no animals were harmed in the making of it, and that none of the fish discussed herein, now both deceased, were eaten by me or anyone else.
I have owned two Bettas in my life, and for all that, I really couldn't comment empirically on their average lifespan. The first one lasted roughly six weeks before its distended belly was pointing upward, and its demise so colored my impressions of Betta ownership that I didn't even bother paying attention to the date the second one came home from Petco with me.
The second one survived its uncounted months largely in a pool contaminated by its own sewage. I fully admit fault here. I neglected my fish. But despite living in water that was commonly colored a sickly shade of green, this second Betta exhibited a hardiness I wouldn't have thought possible outside of a wild pack of ravenous wolves. I used to double up his feedings on the days after I had forgotten to feed him in the hopes that his 2-inch metabolism might regulate itself in the aggregate, a lot like skipping the cable bill in January and paying twice as much just as the shutoff notice arrives in the mail around Valentine's Day.
There is no wikipedia entry on relationships with me, but I can certainly report that they have an average lifespan of almost exactly 18 months. With proper care and feeding, they can live to the ripe old age of 2 1/2 years. But even with my best attempts at regular maintenance, and even with repeated changings of the water, I haven't ever managed to make them live any longer. So my second Betta fish and I were forced to pack up our shit and go.
The day before the fish and I left New Brunswick, New Jersey for what would prove to be his final time, I changed his water. The next day, I wrapped his bowl with Saran wrap, poked holes in the top of it, and suspended him as best as I possibly could away from my torso for the hour-and-a-half trip to the storage space in Glen Rock, New Jersey that currently houses thousands of dollars worth of my possessions. A little later, I brought the fish to his new living quarters on top of my mother's mantel, and less than a week after that, I took the unprecedented step of changing his water again. He wasn't looking particularly well. I thought two water changes in two weeks might do him some good.
But I guess some things are beyond saving. The fish died on a Sunday morning. He died far away from the aged white kitchen walls where he spent the lion's share (or Siamese fighting fish's share?) of his days swimming semi-contentedly through sage-colored water. He died at an unknown age, which I would speculate was somewhere in the neighborhood of eighteen months. But you never know; he might have lived 2 1/2 years.