I found a way to reset the duration of the tree of life.
Today I was in Drak Tharon and on the last boss you get transformet into a skelet. After I got my flesh restored the duration of my tree of life was resetted to 30 seconds. I don't know any other place atm where you get transformed b/c normally the tree form prevents any kind of shapeshifting so it isn't this useful, but maybe someone else knows some occasions where this could work.