"Poseidus's spawn time seems to range from 72-120 hours and is subject to server restarts, meaning his spawn resets after a maintenance or the weekly restart on Tuesday at 5:00 am.
The weekly maintenance usually ends at 11:00 am server time, meaning he won't spawn till at least Friday at 11:00 am server time and at the latest 11:00 am Saturday.
He will only spawn 1-2 times in an ENTIRE WEEK, so be prepared to never have your wish of getting one fulfilled. If there is a maintenance at some other point besides Tuesday in the week, he may only spawn 0-1 times in that week.
Don't waste your time trying to farm him on Tuesday, Wednesday, or Thursday, because he will NEVER spawn on these days as long as there is a server maintenance on Tuesday. If he is killed on Friday, he will not spawn on Sunday at all, but may spawn on Monday/ Tuesday morning if killed just before 5:00 am Saturday morning. If he is killed past 5:00 am Saturday morning, he will not spawn again until Friday.
So, if you are hoping to kill Poseidus, get his precious loot, and waste as little of your time as possible, pay attention. Only camp his spawning points on Friday at 11:00 am to Sunday at 11:00 am, if you do not find him at some point during those 48 hours, he was found and killed by someone else. If you wish to take a swing at your luck, camp his spawning points from 11:00 am Monday morning to 5:00 am Tuesday morning. There is no guarantee that he will spawn on Monday or Tuesday, and there is only a small chance he will spawn on Monday or Tuesday if he was killed in between 11:00 am Friday to 5:00 am Saturday.
Good luck farming guys and remember that he usually doesn't take 120 hours to spawn, and will most likely spawn after about 96-110 hours as is what the majority of the reports show.
If you think it would be a waste of time farming him, remember that he has been sold in prices ranging from 30k gold to 100k gold, with 100k gold being more common. If you happen to find one being sold for a small amount like 30k, BUY IT AND SELL IT FOR MORE!
In addition to the 270% swimming speed mount that can be used anywhere, he also has a chance to drop one of these three enchants: Formula: Enchant Bracer - Major Strength, Formula: Enchant Bracer - Agility, Formula: Enchant Bracer - Mighty Intellect. He may also drop a 359 BoE world drop as well.
The only downside would probably be that he is hostile. This means that if you are camping his spawn and happen to be AFK... you may be killed by the easiest of the rare mobs. If you don't become a sociopath or fall into deep depression after this, then maybe you can get over your humiliation and continue trying!
Oh yes, and I should mention that you CAN NOT SKIN HIM, so I'm just throwing that out there if you wanted a chance at some skin... even though it really doesn't matter since you most likely won't be trying to farm him for some savage leather.
Also, his corpse despawns after a short three minutes. Consider it useless information if you wish.
In conclusion, killing this rare takes hard work and luck, but the payoff is like nothing ever seen. We are talking tens of thousands to over a hundred thousand gold here.
THIS IS THE BEST RARE ADD EVAR! TAKE ALL THE LOOT!
Good luck guys."
According to wowhead.
