A long time.

Over the course of the next few years, Ning continued his closed cultivation. Since the entirely focused on his cultivation, in just a couple of years, he broke through to the next realm. 

Once he broke through, he started going into timer-based energy absorption. It was incredibly mentally exhausting to go in and out of the body every single time, but slowly, after a few years, he got used to it. 

Once enough time had passed, he left the closed cultivation. He went around the sect, met the old people that were there, and met the new people that were there. 

Reya and Nilo had already left the academy, along with many, many of the students Ning knew. Similarly, many others had come to join as well. The world was vast, and there would always be enough students to fill the academy.