Reference:

"The Sutra of Bodhisattva Ksitigarbha's Fundamental Vows"

There was in the past, an inconceivable number of kalpas ago, a Buddha bearing the title of ENLIGHTENMENT-FLOWER-SERENITY-SELF-SOVEREIGNTY-KING TATHAGATA. The life-span of that Buddha was four hundred thousand million asamkhyeka kalpas.