Bhagavad Gita Chapter 8, Verse 17

सहस्रयुगपर्यन्तमहर्यद्ब्रह्मणो विदु: | रात्रिं युगसहस्रान्तां तेऽहोरात्रविदो जना: ||

sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ rātriṁ yuga-sahasrāntāṁ te 'horātra-vido janāḥ

Those who know the day of Brahma, which lasts a thousand yugas, and the night, which also lasts a thousand yugas, understand the concept of Brahma's day and night.

Krishna describes the vast time scales: 'Sahasra-yuga-paryantam ahar yad brahmano viduh'—those who know (viduh) the day (ahar) of Brahma (brahmanah), which lasts (paryantam) a thousand (sahasra) yugas (yuga). 'Ratrim yuga-sahasrantam te 'horatra-vido janah'—and the night (ratrim) which also lasts a thousand yugas (yuga-sahasra-antam), those (te) people (janah) are knowers (vido) of day and night (ahoratra). This verse introduces the cosmic time scale: Brahma's day lasts a thousand yugas, and his night also lasts a thousand yugas. This is the vast cycle of creation and dissolution. Understanding this helps you realize the scale of material existence—even the highest material worlds exist within these vast cycles. This sets up the teaching about the cycles of creation and dissolution (verses 8.18-8.19).