I had a big yellow haddoni for about 7 years before I gave it away. The one I have now is going on 6 years.
I also have a H. crispa that I have around 14 years, but that is not was you asked.
Daphne Fautin stated in her book that host anemones can live hundreds of years, but her hypothesis was based on observations that weren't really valid. A couple of her observations were that small anemones were rarely seen (turns out she wasn't looking in the right places) and that anemones grow very slowly (as we all know, anemones can grow very quickly in the right conditions).
In short, they are certainly capable of living a long time, but whether or not hundreds of years is realistic has yet to be shown. Even large anemones have natural preditors. I wouldn't think the odds would be in favor of them living hundreds of years.
A DNA study might help. Higher animals have sections of their DNA that get shorter as the DNA divides. When the sections get too short your cells stop functioning properly and you die of old age. This is why making a clone from an adult human is not really practical. For example, Dolly the cloned sheep, died of old age at the age of three because the DNA that they used was from an adult sheep.
If anemones don't have those DNA sections then they could live indefinately.
