So in short, the gods are desperate and need some STRONGEST DISCIPLE to clean the world of mess, because gods do not dirty their own hands.

A typical classy trope of isekai to be a whore hero, but it works. Start was a bit a mess with the spokesman stating they are summoned for their modern knowledge, but proceed to sparta train them, and cherry on top of boi baited with BE THE ERO YOU ALWAYS WANTS TO BE.
I don't think it is mentioned how long he has trained.
Let's say the year he is isekai as year 0. Woman tells him there was 1 15 years ago, then say add another 1 in 10 years before that, that they are the adept they get in his batch, and now he is the latest addition. I'd say MC trained for longer than 25 years.
I wonder what happened to the OVER 9000 adepts they send to save the world. Taking into account 10 years is fastest to make a proper adept, means its been 90 millennia of the problems. How the hell the people/civilization still persist in that very long time?? Verily an absurd plotting.

Next, what happened to thousands giving up and leave? Leave where?? They are trapped here just to suffer.