I don't know how long anemone can last in captivity either. For some reason, I think of anemone as a clicking time bomb. Time will only tell when your tank will crash, you know? But I don't know, I just heard of bad stories about some tanks with anemone. And when they crash, it's like the Nintendo Entertainment System: when it crashes, you start from the beginning of the game again.
I don't know if you want to here this answer or not, but here goes.......
I had a really healthy RBTA that I bought because my clown was hosted in it at the store. I brought it home, acclimated it, and put it into the tank. it was doing well, then one day decided to move. It got to close to my frogspawn and the frogspawn zapped it. after that, it pretty much moved around the tank at will until eventually, it got pulled into a PH
and chopped up rather nicely(and yes I had a screen on the PH
). I had to do a truckload of water changes and it took about 3 months to get my levels right again.
then I bought another anemone for my clown. this time a long tentacle. it died in a matter of weeks. it stunk my house up and clouded the tank. again, it took forever for me to get my water right again.
the moral of my story? nothing really, just my experience with anemones. Some people have good luck with them and other people have nothing but headaches. I will more than likely never get another one.