Several years of a living hell to be exact. Many don't survive the training beyond the first week.

[And it takes years so... Enormous amount of discipline? That's kind of an understatement.]

Well, as I said even back at my own world most of the world remains ignorant of it so I'm not surprised. It's useful but it's also a huge power, not many would give it a good use, so it's better to keep it as a secret in the shadows.