After Shin Uchiha had spent nearly a month training with Sasuke in the illusion, subjecting him to almost a month of grueling punishment, Sasuke Uchiha finally managed to control his chakra steadily without making mistakes during training. At that point, he had essentially mastered the first stage of Lightning Release Armor.
The next step was to train in the real world, then keep training and stimulating his body so that it could grow stronger quickly while also improving his reaction speed.
The Fourth Hokage had been so powerful because of his Space-Time Ninjutsu-Flying Thunder God-and his own superhuman reaction speed. The combination of those two was what had made the Fourth Hokage so renowned.
Just because Sasuke had learned the first stage of Lightning Release Armor in a month did not mean it was easy to learn. Shin Uchiha and Sasuke Uchiha had trained without sleep or rest throughout that month. Since they were inside an illusion, they could train without restraint, unafraid of mistakes, bodily injury, or death. Beside Sasuke, his teacher, Shin Uchiha, had also stood watching him like a hawk with the Metal Rod in hand, keeping him constantly on edge.
Combined with Sasuke's considerable talent, that was what had allowed him to complete the first stage of Lightning Release Armor training within a month.
In fact, for an ordinary ninja, even the first stage of Lightning Release Armor could offer lifelong benefits. After mastering that first stage, working hard, and honing their body, becoming a Jōnin would be well within reach-and a Jōnin with exceptional movement and reaction speeds at that. As long as such a ninja was not surrounded or recklessly courting death, their chances of survival would be very high.
With three days left before the month was up, Sasuke Uchiha successfully practiced Lightning