This guy has a respawn of approx. 2hrs.
Took 3 rangers all at the correct stage of the 1.5 pre-quest. Killed him and he dropped 1 stone.
Re-logged the other 2 and no re-spawn/trigger so kept checking back every 30min or so until he was up again, which was about 2hrs 15m later - so I'm assuming respawn is 2 hours.
Good luck!