Master the Elements to win against the experts of those Elements... That was the challenge that Gabriel was given if he wanted to clear the 99th Floor.
The challenge also came with more restrictions than just the one about using a specific element. Another restriction placed on him was that he couldn't take anyone's help. He was allowed to use a Numen, but only one! It didn't matter how many Numens he possessed, but he could only select and use one in the battle.
Other than these restrictions, there was not much that was placed on him. There wasn't anything related to how much time he could take. It all depended on him. He could take one month to learn, or he could train for years. It was all on his capabilities.
