At Sāvatthi.

Then, a certain bhikkhu approached the Blessed One. Having approached him, he paid homage and sat down to one side. Sitting to one side, that bhikkhu said this to the Blessed One: "Venerable sir, how long is an aeon?"

"An aeon is long, bhikkhu. It is not easy to count it as so many years, or so many hundreds of years, or so many thousands of years, or so many hundreds of thousands of years."

"Then, is it possible to give a simile, venerable sir?"

"It is possible, bhikkhu," the Blessed One said.

"Suppose, bhikkhu, there were city made of iron, a yojana long, a yojana wide, and a yojana high, completely filled with mustard seeds, packed tightly into pellets or balls. At the end of every hundred years, a man would remove one mustard seed from the city. Sooner, bhikkhu, that vast collection of mustard seed pellets would be exhausted by this process, but not so an aeon. So long is an aeon, bhikkhu. And of aeons of such length, we have wandered through so many aeons, so many hundreds of aeons, so many thousands of aeons, so many hundreds of thousands of aeons.

For what reason? Bhikkhus, this cyclic existence is without a discoverable beginning. A first point is not discerned for beings who, obstructed by ignorance and fettered by craving, run on and wander in this cycle of existence. For such a long time, bhikkhus, you have experienced suffering, agony, and calamity, the cemetery has been filled up with your bones. Therefore, bhikkhus, it is enough to become disenchanted, to become detached and to become free from all conditions."