CC Madhya 20.305
āpane īśvara tabe aṁśe ‘brahmā’ haya
“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.