Black Rock Depths are a huge Mega Dungeon in World of Warcraft, while i would like to have something like that in FF14...that would be a singular exceptional dungeon. Even for WoW its unqiue in scope.
Just so you can imagine that would be more like as if a Area Map would be a Dungeon you fight trough, with puzzles, diverse quests inside of the dungeon (quests inside of dungeons is normal in WoW), encounters and Bosses.
Also of course its not just a corridor.
According to a quick google search the average completion time is 4-6 hours but that assumes you do everything.

But again while i would like to have something akin to that it would be a unique thing in FF14 aswell and is far far far beyond the scope i envisioned xD