Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 18

Sanskrit:

मैत्रेय उवाचकृतं त्रेता द्वापरं च कलिश्चेति चतुर्युगम् ।दिव्यैर्द्वादशभिर्वर्षै: सावधानं निरूपितम् ॥ १८ ॥

ITRANS:

maitreya uvācakṛtaṁ tretā dvāparaṁ cakaliś ceti catur-yugamdivyair dvādaśabhir varṣaiḥsāvadhānaṁ nirūpitam

Translation:

Maitreya said: O Vidura, the four millenniums are called the Satya-, Tretā-, Dvāpara- and Kali-yuga. The aggregate number of years of all of these combined is equal to twelve thousand years of the demigods.

Purport:

The years of the demigods are equal to 360 years of humankind. As will be clarified in the subsequent verses, 12,000 of the demigods’ years, including the transitional periods which are called yuga-sandhyās, comprise the total of the aforementioned four millenniums. Thus the aggregate of the above-mentioned four millenniums is 4,320,000 years.