Bhagavad Gita Chapter 8.17
sahasra-yuga-paryantam
ahar yad brahmaṇo viduḥ
rātriḿ yuga-sahasrāntāḿ
te ’ho-rātra-vido janāḥ
Translation
By human calculation, a thousand ages taken together form the duration of Brahma’s one day. And such also is the duration of his night.