first group killed him on thrusaday(night?) then our group kill him on saturday morning then today(monday) in the morning we saw him but an epic battle between horde and ally had occured and they won the fight and slayed galleon.
so i say about 2 day??? i still havent confirmed if you can loot him more than once without using the elder coin, because the respawn timer on him is much much longer than sha of anger(2 hours for him)
Just killed him a second time in a week - he does give loot every kill.
1-3 days seems to be the respawn timer, 0-6 hours after server restart he seems to spawn, Instant respawn on server shut downs.