Two years

Aizen skimmed through the Shadow Clone Jutsu, mastering it within the hour.

"Shadow clone Jutsu"

Two Shadow Clones materialized before him. A child's play compared to Healing Jutsu. He tasked one clone with formulating a plan to absorb natural energy for the first time. The other was to act as a test subject, while Aizen himself observed meticulously. The experiment commenced. Following the Shinigami method of absorbing spiritual energy, one clone focused its being, drawing in the surrounding natural energy and channeling it inwards. Aizen observed as the clone's perceived soul power increased. This power, in turn, seemed to nourish the clone's body, subtly enhancing its physical capabilities.

A glimmer of triumph ignited in Aizen's eyes. It appeared the Shinigami method of cultivating Reiatsu could indeed be adapted for manipulating natural energy in this world. However, just as elation began to take hold, a horrifying twist unfolded. The clone's expression contorted in pain. A chilling sight – its body began to petrify, a layer of stone-like material spreading rapidly. With a resounding crack, the clone shattered, fragments of stone and smoke flying in all directions. Aizen watched, a grimace twisting his features. A wave of relief washed over him – good thing he resisted the urge to experiment on himself.

The consequences could have been disastrous. Aizen's face hardened as the Shadow Clone shattered. Even after its demise, a faint connection pulsed, attempting to transfer the clone's memories and energy back to him. With a swift flick of his wrist, Aizen severed this connection using his Zanpakuto, Kyoka Suigetsu. The residual energy and fragmented memories dissipated harmlessly. While losing valuable data was a setback, it ensured his safety. By severing the link, Aizen could continue using the Shadow Clone Jutsu without the potential for a catastrophic feedback loop.

Every action had its consequences. Despite the experiment's failure, Aizen remained undeterred. "The initial attempt yielded no fruit, but the initial absorption of natural energy confirmed the potential of this path," he reasoned, analyzing the results. "The key lies in refining the method." His keen intellect focused on the flaws. "A flicker of odd energy manifested within the clone's spiritual body… perhaps the absorption route requires modification. It should be rerouted to…" A determined glint flickered in Aizen's eyes. He retreated to his study, emerging an hour later, looking slightly drained.

"Observe and analyze the experiment with the same meticulousness I employed," he instructed a new Shadow Clone, handing him a meticulously crafted plan. "Identify and address any shortcomings that arise. Time is a precious commodity for me. You will continue to refine these experimental plans."

"Don't worry, I understand," the Shadow Clone replied with a confident smirk, mirroring Aizen's own. "Excellent," Aizen responded with a curt nod. He then formed another Shadow Clone, setting both to work under the first clone's guidance, meticulously following the revised training plan. As the Shadow Clone meticulously practiced the refined technique, Aizen's focus remained unwavering. The first obstacle emerged, and Aizen watched intently as the clone navigated it successfully. Relief flickered across his face, a small victory in the grand scheme. But just as quickly, another explosion ripped through the room, showering the floor with debris.

Aizen remained impassive. The first failure had steeled him for a long, arduous journey. "Each failure reveals a new challenge," he murmured, his voice steady. "Progress will be incremental, a step-by-step process of refinement." His mind churned with possibilities. Beyond simply absorbing natural energy, Aizen envisioned a future where he could integrate it with the world's unique chakra system. The legendary power it promised – immortality – was a potent motivator.

"My approach to natural energy deviates from the norm," he acknowledged, a sigh escaping his lips. "Merely acquiring it isn't enough. Uniting it with chakra is akin to forging a whole new system. The road ahead is daunting." However, a steely resolve hardened his gaze. "Survival in this ruthless world demands it. I will not be a pawn to pacifists or outside forces. I will be the master of my own destiny, and for that, I need unparalleled power!" Aizen's eyes gleamed with a fanatic's fervor. "Immortality!" he declared, the word resonating in the stillness of the night. "With this power, victory is assured. No obstacle, no hardship, will deter me!"

Exhausted from a day filled with experimentation, Aizen sought a moment of rest. He stretched out on a living room sofa, then ascended to his balcony, overlooking the East Street. Even amidst the war, the avenue pulsed with life, its vibrant lights and bustling crowds a stark contrast to the solitude within him. Food vendors hawked their wares, children frolicked with their parents, and elderly folks strolled leisurely. Their faces, devoid of wartime anxieties, held an unwavering hope for a better tomorrow. Ordinary, carefree, and truly happy. A wistful sigh escaped Aizen's lips. "A beautiful village, Konoha," he murmured. "Yet, it harbors no place for me." The next day, Aizen returned to the medical department, diligently applying himself to his studies while simultaneously overseeing his Shadow Clones' experiments. He neglected none of his pursuits, a routine that continued day after day, year after year, for two whole years.