A certain bhikkhu asked the Buddha, “How long is an aeon (kappa)?” The Buddha explains with a simile:
If a man were to take once in one hundred years one seed from a heap of mustard one league in length, breadth, and height, he would come to an end of the seeds before one aeon is passed. Incalculable is the cycle of existences (saṃsāra). S.ii.182.