Sāsapa Sutta (SN 15:6)
Dwelling near Sāvatthī. Then a certain monk went to the Blessed One and, on arrival, having bowed down to him, sat to one side. As he was sitting there, the monk said to the Blessed One, “How long, lord, is an eon?”
“Long, monk, is an eon. It’s not easy to count as ‘so many years’ or ‘so many hundreds of years’ or ‘so many thousands of years’ or ‘so many hundreds of thousands of years.’”
“But is it possible to give an analogy, lord?”
“It is, monk,” said the Blessed One. “Suppose there were an iron fortress—a league long, a league wide, a league high—full of mustard seeds packed tight, and a man would come along once every hundred years and take from it a single mustard seed. More quickly would that great heap of mustard seed waste away and be consumed by that effort, but not the eon. That’s how long, monk, an eon is. And of eons of such length, not just one eon has been wandered-through, not just one hundred eons have been wandered-through, not just one thousand eons have been wandered-through, not just one hundred-thousand eons have been wandered-through.
“Why is that? From an inconceivable beginning comes the wandering-on. A beginning point is not discernible, though beings hindered by ignorance and fettered by craving are transmigrating & wandering on. Long have you thus experienced stress, experienced pain, experienced loss, swelling the cemeteries—enough to become disenchanted with all fabrications, enough to become dispassionate, enough to be released.”