SB 3.11.18
maitreya uvaca
krtam treta dvaparam ca
kalis ceti catur-yugam
divyair dvadasabhir varsaih
savadhanam nirupitam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Maitreya said: O Vidura, the four millenniums are called the Satya-, Treta-, Dvapara- and Kali-yuga. The aggregate number of years of all of these combined is equal to twelve thousand years of the demigods.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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-sandhyas, comprise the total of the aforementioned four millenniums. Thus the aggregate of the above-mentioned four millenniums is 4,320,000 years.