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SB 3.11.18

maitreya uvāca
kṛtaṁ tretā dvāparaṁ ca
 kaliś ceti catur-yugam
divyair dvādaśabhir varṣaiḥ
 sāvadhānaṁ nirūpitam
Synonyms: 
maitreyaḥ uvāca — Maitreya said; kṛtam — the age of Satya; tretā — the age of Tretā; dvāparam — the age of Dvāpara; ca — also; kaliḥ — the Age of Kali; ca — and; iti — thus; catuḥ-yugam — four millenniums; divyaiḥ — of the demigods; dvādaśabhiḥ — twelve; varṣaiḥ — thousands of years; sa-avadhānam — approximately; nirūpitam — ascertained.
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.