Problem is, this rare mob location is next to conquest/honor pvp gear vendor and he is getting killed instantly as soon as he spawn.
Some of the rares are extremely "hard" to kill unless camped because of their location near crowded places such as popular questing areas. Fortunately the respawn timer is around one hour as mentioned above so it's pretty easy to camp it. Getting the wanted vanity item is another matter though as the drop chances are around 10-20%.
I have 5 of the Elixirs, although I don't really "camp" him so to speak. I tend to play a lot during down time (i.e. 1 AM) and just happen to see him around when I fly around Dread Wastes doing dailies or as a slight detour from Golden Lotus stuff. He's surprisingly easy to find alive vs. Scritch who drops the Peace Pipes. I've killed him maybe 15-20 times and gotten 5 of the potions, one of which was wasted 5 minutes after popping it due to a nasty Elite surprising me as I exited a failed dungeon. His spawn timer is something like 60-90 minutes.
I noticed that too on several occasions. I'd say it's safe to say that all rare spawns in one zone will always respawn at the same time.
For respawn timers - i've seen some rares respawn ~45 minutes after they've been killed, while the longest time i sat on one spawn was about 2 hours after a kill.
If you had to wait two hours it simply means that the specific mob you were after was not a part of the previous spawn. As I said, it's random which ones spawn and which ones don't, therefore there is essentially no guaranteed respawn time on any specific rare mob but more so the rares for any zone as a collective.
It's essentially a more refined concept of shared rare timers similar to the Drakes in Storm Peaks (TLPD and the blue Drake) in that they have a set timer but it ultimately comes down to RNG.
I have gotten a total of two flasks, one of which I only got about 15 minutes worth before being killed -.- The second flask I have I am holding onto for when I hit level 80, I plan on burning through northrend at record speeds on my monk. Hoping to get another two flasks before I seriously level him, be nice to do Outland + Northrend in about 2 hours :3