Ryuhei didn't know how long it took him, but that time was absolute hell by all means. Everything that the Dragon Slayer thought about magic was proven wrong as soon as teaching began. Firstly, he was sorely mistaken when he thought that learning magic was easy. As a matter of fact, learning how to use the magic was by many means harder than learning how to use spells! It took Ryuhei about two weeks just to learn how to use a demon's wind. The first week, the Sky Demon had to force him to stop using lightning and water, which was torture since he got stung every time he even conjured up one of those elements. Now that he finally stopped using those elements, the second part was learning how to control and channel the wind. Yeah, Ryuhei didn't do so hot with that. In other words, he was a walking air conditioner for about five days. The two days after that were filled with absolute and sheer laziness on Ryuhei's part: He wanted to do not a damn thing for two days straight.
When that was all well and done, and Ryuhei wanted to put in work again, then came the spells. Now it was easier to learn the spells than it was to learn the magic, but that didn't mean it was easy. The first spell he was to learn was something called Aura. It was a spell that generates a steady wind around your body, so it can protect you from attacks and all that jazz. One can imagine how hard it was just to learn that spell. It was just like learning how to channel wind: It took him about three days and he became a walking AC. The fact that someone could learn how to channel wind energy, and then bomb at trying to make an aura of that energy is honestly retarded, but he learned eventually. Though it honestly beat Ryuhei how someone was going to need an aura of wind enveloping their body.
The second spell, now that was something that Ryuhei created himself. One morning, he channeled his wind power into his right arm, which ended up generating a tornado-like covering which engulfed his arm. He swung his arm once, and at once the trees that were in the arm's direction dropped with one fell swoop. It took about a couple of minutes after that for Ryuhei to realize that he had just created a spell that creates essentially a sawblade out of air that can be used as a sword. He decided to call this spell "Crankshaft", for no reason whatsoever. He certainly didn't know too much about cars. Either way, the Dragon, or rather, Demon Slayer was proud of what he had done, even though he was almost ashamed of himself for the conditions of which he began learning the magic, which really shouldn't need to be brought back to.
In fact, the truth was that Ryuhei wanted to get out of there as soon as possible. He literally couldn't handle every night of ...conduct that was happening. Some might think that the Devil Slayer was a prude by this, but the truth was that his body nor private parts couldn't take it, basically. After about three weeks, he started to feel tired as hell. It was to the extent where he could barely get out of bed in the morning. He would have to get the hell out, sooner or later. Or he would die by Snu-Snu. Literally.
A morning ended up exposing a chance for Ryuhei. When he woke up, he found it was raining. And he also found out that the demon wasn't keeping her usual vice grip on him. As he got out of the makeshift bed, his heart nearly jumped out of his chest as he heard her groaning. Okay, he thought. I really have to get the fuck out of here, and fast. He made a beeline for the bush in which he entered, and ended up slicing it down with the Crankshaft. As soon as the foliage was out of his way, he flexed faster than a man would if he found out he knocked some chick up.
He turned back not once to that clearing, and made to erase it from his mind. But, unlike most everything that's happened to him, he was actually unable to forget it. Like the narcolepsy decided not to work on this or something. In other words, he was stuck with the reminder that he almost got sexed to death just to learn a goddamn magic. Which is why he'll rarely ever talk about it, if at all if one were to ask him how he obtained his magic.