Two days. Zero took longer than he expected to create the world flower guardian. The first two souls were destroyed in the process in an unfortunate accident. Zero misjudged the number of chaos fragments used and the experiment went south when he added his 'thoughts' to the unstable mixture.
Then, after adjustment and calculating the right amount of chaos fragments that a regular soul can take, he realised that he had been doing it wrong. While the proportion was now correct, the souls still disintegrated because they couldn't accept the burden of the 'thought' in the reforge.
After consulting several experts, Zero finally understood why he failed. The souls had to undergo enough torture and pain to toughen enough so that they wouldn't crumble under the pressure of the thought's responsibility. Not every soul can accept the position of the world flower's guardian and the duties that came with it.
