I'm surprised Ifrit took so long compared to Moogle... I suppose many more people were still learning how to do hard fights then and it provided good technical skills for Moogle. I think people are getting better and better and there will surely be people ready to blow through this quest as soon as it comes out so I don't think it will take too long...

My guess: 5hrs 15mins

Come on hard core end gamers, do it in less! I'm sure you can! Go, go, go! (Once patch is out XD)

Main thing I expect will be holding people back will not be the difficulty of the battle but rather the server instability as everyone rushes to crash it. Poor servers will suffer...