Thanks to her motivation to see Shiba and the hellish training environment provided by Lina, Eini needed less than a month and a half to master the Water Law.
She was just a step away, which could be done in an instant or last for several years, but Lina was still pleased with Eini's speed.
Eini's temperament also improved after she mastered the Water Law, so she controlled herself better once she was allowed to see Shiba again.
Although Lina felt that Eini would have tempered herself better if she remained training for several years, she promised to free her after mastering the Water Law, so she did it. She felt that if she pressured Eini too much, it would be harmful to her.