I read a post by nechin on Wowhead.
If you have it on group loot it is 100% to drop once, and rolled on by all.
If you have it set to personal loot, the chance is split between all players. So five players each get 20% chance to get the drop. Which means there is a decent chance that no one will get it.
If I recall his post correctly, this is how it breaks down:
~ 33% that no one gets one.
~ 41% one person gets one.
~ 20% two people get one.
~ 5% three people get one.
< 1% four people get one.
< .01% all five get one.
There is math behind this, makes my head hurt to think about it.
I am just relaying what I read, I have no concrete evidence that this is how it works, but it does seem reasonable given that there is indeed a chance for no drop at all.
That being said, if you could get a group who would consistently do this dungeon till everyone gets one your odds would get better and better as each member has one. Problem comes when someone gets one, can you count on them to keep coming. You would need to be pretty decent friends or a very honest stranger such as myself
to plan this with. I would say do group loot then for sure you would only need to kill him 5 times. In personal loot there is potential to go less than 5 kills but also potential for more than 5 kills.