Roughly had 7 years passed as Yuji regressed and fully realized that it wasn't a dream but rather reality. As a boy in his past life he was weaker than the average "E Rank Hunters" and experienced death multiple times that if it weren't for the healers and potions he would have died a long time ago.
But now seeing as he knows the future and the events that will unfold in the coming 30 years when the demon king is brought back to life by Gods to destroy mankind and stop his rise to power, he must train everyday and cultivate his mana before he leaves for the academy which he will be evaluated by the schools administration and based on his rank will determine the class he's in.
Yuji spent 2 years training and cultivating his mana with the knowledge he regressed with making him rank up to A Rank, but he knew no one could find out about him being an A Rank before he reached the peak of necromancy.
His Necromancy skills leveled up that he could summon the dead even if by just a breath from his mouth, little by little had he started regaining his lost powers of the Undead.