|Madhya-līlā||Chapter 20: Lord Śrī Caitanya Mahāprabhu Instructs Sanātana Gosvāmī in the Science of the Absolute Truth|
Bhaktivedanta VedaBase: Śrī Caitanya Caritāmṛta Madhya 20.305
kona kalpe yadi yogya jīva nāhi pāya
āpane īśvara tabe aḿśe 'brahmā' haya
kona kalpe — in some lifetime of Brahmā; yadi — if; yogya — suitable; jīva — living entity; nāhi — not; pāya — is available; āpane — personally; īśvara — the Supreme Lord; tabe — then; aḿśe — by His plenary expansion; brahmā haya — becomes Lord Brahmā.
"If in a kalpa a suitable living entity is not available to take charge of Brahmā's post, the Supreme Personality of Godhead Himself personally expands and becomes Lord Brahmā.
One day of Brahmā consists of the four yugas multiplied a thousand times — or, according to solar calculations, 4,320,000,000 years — and such also is the duration of his night. One year of Brahmā's life consists of 360 such days and nights, and Brahmā lives for one hundred such years. Such is the life of a Brahmā.
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His Divine Grace A. C. Bhaktivedanta Swami Prabhupāda, Founder Ācārya of the International Society for Krishna Consciousness