Elemental Apotheosis:

[archest]

On Hit with an Electric Spell to an enemy immune to Electric Damage: for air savant .
They become vulnerable for the next strike

do they need to hit 2 times to remove immunity?

hit 1 time remove immunity of next strike.
hit and get damage
hit again to remove immunity of next strike.
hit and get damage


?
or is there a permanency to it
or a time limit to it?
after the 1st strike.

[YUTANG75]

Your first hit on an immune enemy applies a short duration debuff which removes their immunity.

[Tyrande]

First strike should be a low powered AOE spell just to strip immunity like Electric Loop.

Then cast Ball Lightning or Chain Lightning or Thunderstroke Maximized, Empowered, Intensified, Heightened, Quickened. As sorcerers we cast these so fast anyways.

[WruntJunior]

Your first hit on an immune enemy applies a short duration debuff which removes their immunity.

This. In addition, this immunity stripping applies for everyone else, which makes sorcs, alchs, and druids nice to have around for casters of the same element spells that don't have immunity stripping.