Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 19

Sanskrit:

चत्वारि त्रीणि द्वै चैकं कृतादिषु यथाक्रमम् ।संख्यातानि सहस्राणि द्विगुणानि शतानि च ॥ १९ ॥

ITRANS:

catvāri trīṇi dve caikaṁkṛtādiṣu yathā-kramamsaṅkhyātāni sahasrāṇidvi-guṇāni śatāni ca

Translation:

The duration of the Satya millennium equals 4,800 years of the years of the demigods; the duration of the Tretā millennium equals 3,600 years of the demigods; the duration of the Dvāpara millennium equals 2,400 years; and that of the Kali millennium is 1,200 years of the demigods.

Purport:

As aforementioned, one year of the demigods is equal to 360 years of the human beings. The duration of the Satya-yuga is therefore 4,800 × 360, or 1,728,000 years. The duration of the Tretā-yuga is 3,600 × 360, or 1,296,000 years. The duration of the Dvāpara-yuga is 2,400 × 360, or 864,000 years. And the last, the Kali-yuga, is 1,200 × 360, or 432,000 years.