There appears to be a max number of hits as well as time limit on this spell. I was playing with it at the farm in Dragon Scale Hills on a group's tank there. When he had several mobs hitting him, it wore off quite quick. When he was singling them seemed to go near the max duration.
If I get a chance to log it, I'll try and determine what the max hit is. So it's still a great spell, but not nearly as juice as I first thought.