Bhagwad Gita
Chapter 8, Verse 17
सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदुः।
रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जनाः।।8.17।।
By human calculation, a thousand ages taken together is the duration of Brahma's one day. And such also is the duration of his night.