Not sure what the life span is, probably somewhat dependent on the species as well. I have read that in the wild they can live quite long if something doesn't come along and predate them.
From my own experience, I've had a BTA for approx a year and a half and it's doing well hosting a pair of Maroon Clownfish. There have been some issues but currently it's growing actively and appears healthy. I've read many threads on various forums where keepers of nems have had great success in maintaining their health, to the point where they are propagating through splitting. Check the following link for a lady with vast experience in raising anemones, primarily RBTAs.
http://www.karensroseanemones.com/index.htm