One Year

Emir was cultivating with the white Aether Core nestled between his crossed legs.

Gradually, the condensed Aether within the core seeped out and was absorbed into his own.

It was a slow process, as he had to absorb the Aether, convert it into his favorite aspect, the Weaver's Veil, and then let it flow out, returning to the atmosphere.

He was 'breathing,' as Sofia said during his class.

Several hours had passed since he began, yet the Aether core remained unchanged, a common occurrence for those who practiced the method in this way. 

And yes, there are two ways to absorb an Aether Core. 

One can either do it patiently and precisely, like Emir, or do it at once, stuffing their own core full to the brim and beyond, forcing it to digest, absorb, and grow.

Obviously, the first way would gain the Celestial better results. That was not only due to the leeway given to their Aether Core but also due to the Embodiment method.