Pabbatasutta

sn15.5:1.1 At Sāvatthī.

sn15.5:1.2 Then a mendicant went up to the Buddha, bowed, sat down to one side, and asked him,

sn15.5:1.3 “Sir, how long is an eon?”

sn15.5:1.4 “Mendicant, an eon is long.

sn15.5:1.5 It’s not easy to calculate how many years, how many hundreds or thousands or hundreds of thousands of years it lasts.”