Ultima skill when?

[Taranok]

Uh, never? There are story reasons as to why it will forever be unattainable.

It was part of the ARR main scenario as to why it is now lost forever (it was contained within the Heart of Sabik, the Ultima Weapon's power core - Lahabrea cast it to reduce the Praetorium into a pile of molten slag as part of his gambit to shatter Hydaelyn's barrier around the player's character - "bringing the Hammer of Darkness to bear on the Shield of Light", as he termed it. When we destroyed the Ultima Weapon, the Heart of Sabik was also destroyed as it went critical taking the spell, and the Ultima Weapon too, with it).

I suppose as the Ascians are the only ones who know how to cast it, and given the revelations in SHB, there may be a chance of learning it eventually,

but as it stands I think that's highly unlikely (you only have to look at FFXI which had plenty of 'traditional' FF spells, and yet Ultima has never appeared in that game).

This would be true except that

we've fought a creature that very likely is where Ultima came from and Ultima itself as used by Ultima Weapon was likely an imitation of the Ultima used by the High Seraph. A being considered to be a living primal. So it makes sense that Ultima as a "primal" can impart its secrets at least onto Summoner, and if Ultima Weapon could have an artificial Ultima created, then the players could probably copy a real Ultima now.