This is how I heard. At one time, the Blessed One was living in the city of Sāvatthi, in Jeta’s Park, at Anāthapiṇḍika’s monastery. A certain monk approached the Blessed One, worshipped him respectfully and sat down to one side. He asked the Blessed One, “Bhante, how long is an eon?”
“An eon is long, monks. It is not easy to count and say that it is so many years, so many hundreds of years, so many thousands of years, or so many hundreds of thousands of years.”
“Then, Bhante, is it possible to give a simile to know the length of an eon?”
“It is possible monks,” The Blessed One said. “Suppose, monks, there was a city with iron walls ten kilometers long, ten kilometers wide, and ten kilometers high, filled with mustard seeds. At the end of every hundred years, a person would take away one mustard seed from there. As a result of this, the great heap of mustard seeds might be soon taken away, but the eon wouldn’t have still come to an end. So long is an eon, monk. It is not just one eon that was passed by, not hundreds of eons, not thousands of eons, and not hundreds of thousands of eons.
“What is the reason for that? It is because, monks, this cycle of rebirth is endless. The beginning of this extremely long journey cannot be discovered. These beings, hindered by lack of knowledge of the true nature of life and bound by craving, roam and wander on in this endless journey.
“For such a long time, monks, you have experienced various types of suffering, tragedies, and disasters. You have filled the cemetery with your dead bodies. Therefore, monks, the time has come for you to understand the meaningless nature of all conditioned things. The time has come for you to become detached from them. And the time has come for you to be liberated from them.”