From what I can tell this boss has around a 2-3 hour respawn It is not the same every time. I like to camp him on days when I'm just farming bgs. I was lucky enough to get the Crystal of Insanity first try on my mage but, after around 15 other kills, no luck on any other characters, His shadow bolt ability is laughable but be sure to move out of the shadow pools he drops as they will kill you,
The 500 all stats buff from the crystal is amazing to have for anything other than guild progression so you don't have to waste your flasks. It is never consumed and has a 15 min cd with an hour long duration.