Unukalhai: Ifrit, Titan, Garuda, Leviathan, Ramuh, Shiva. Born of the land and returned to it.
Unukalhai: Six primals, one for every elemental aspect. So where does that leave Ravana and Bismarck?
Unukalhai: Earth and fire. Water and wind. Your latest foes are aspected to not one element but two.
Unukalhai: A rather simple but noteworthy distinction, for it allows them to feed upon aether that much more efficiently.
The six Primals had one element, so they had the shortest summoning duration. Bismarck and Ravana had two elements, so they lasted longer. Hence why Bismarck was able to swim(?) among the clouds for a long time.

But what about the Elder Primals Bahamut and Odin? Are they aspected to even more elements or none at all? Is that why they can roam the lands for almost an eternity? Would a being without an element last the longest? Are what the Garleans call Eikons basically what Eorzans refer to as Elder Primals?

What about light and darkness? Are these considered elements? Can a being of light or darkness be summoned as a primal? And would they last even longer?