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SB 3.11.19

catvāri trīṇi dve caikaṁ
 kṛtādiṣu yathā-kramam
saṅkhyātāni sahasrāṇi
 dvi-guṇāni śatāni ca
Synonyms: 
catvāri — four; trīṇi — three; dve — two; ca — also; ekam — one; kṛta-ādiṣu — in the Satya-yuga; yathā-kramam — and subsequently others; saṅkhyātāni — numbering; sahasrāṇi — thousands; dvi-guṇāni — twice; śatāni — hundreds; ca — also.
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.