Gemara: [Rav Shimon and Rav Elazar left the cave and were distressed by people occupied with agriculture.] Everyplace that they looked was consumed by fire. A bat kol (Heavenly voice) called out: "Have you come out to destroy My world? Return to your cave." They went back in and lived there for twelve months. After twelve months, they said: "The verdict for the evil in Gehinom is twelve months."
Ein Ayah: Hashem’s opinion is, of course, inestimably higher than that of any human. Hashem determined that despite all the evil and lowliness that can come from concentration on mundane needs, when life, with its blemishes, proceeds according to the divine plan, light can come specifically through unsightly things. The light can expand to give meaning to such activities and provide light for righteous and straight-hearted people.
One should start with the highest level of sanctity applied to lowly elements of life to maintain them as is. Then, slowly and incrementally, one can purify matters until they eventually reach the goal that the Creator intended for them.
Hashem complained to Rabbi Shimon and Rabbi Elazar, who had not yet reached the level at which they were able to connect themselves on their high level to the low level at which simple people interact. This caused them to want to destroy the lower level so that they could build a higher level on top of the ruins. In fact, they should have strove to improve the world slowly until people’s lives would be perfected and evil would turn into good.
Therefore, they were told to return to the cave to go up to yet a higher level, so that they would have such wisdom and sanctity that they would be able to actually go down to the level of people leading a normal life and fix them the way Hashem wanted. This is in line with thep’sukim: "The world is built on kindness" (Tehillim 89:3) and "The world was not created to be void" (Yeshaya 45:18).
After twelve months, they felt that they had to reach the level of closeness to Hashem’s wisdom at which they would be able to find interest in improving life as simple people experience it despite their own lofty level. This is, after all, how Hashem wants the world to be – repaired according to His attributes.
In order for Rabbi Shimon and Rabbi Elazar to succeed in this task, they needed to change things in the very essence of their spirit. They had to take this outlook of raising life as is to a higher level without eradicating the present situation with some swift revolution, which requires the destruction of the world as we know it. Therefore, they were given the amount of time that it takes to fix the souls that have serious blemishes (in Gehinom) and to turn them into a new essence for the good.