I can't give an exact lifespan. Weve been in Hughes 50 years and they have been there as long as we can remember with no water and sun all day. Also in front gardens for decades. They don't seem to get borers or rust etc. Seedlings develop to keep the clumps going - every few years city parks seem to remove dead branches, etc. I guess they are not particularly attractive when not in flower. We have had one in our front garden for years never watered but pruned back to keep it from growing too high, it happens to grow in semi-shade. Puts on a lovely display every year.