Theories vary, but by traditional astronomical calculation, a mahayuga equals 4,320,000 solar years (or 12,000 "divine years;" one divine year is 360 solar years) - with the
Satya Yuga lasting 1,728,000 years, Treta Yuga 1,296,000 years, Dvapara Yuga 864,000 years, and Kali Yuga 432,000 years.
Mankind is now experiencing the Kali Yuga, which began at midnight, February 18, 3102 bce (year one on the Hindu calendar [see Hindu Timeline]) and will end in approximately 427,000 years. (By another reckoning, one mahayuga equals approximately two million solar years.) A dissolution called laya occurs at the end of each mahayuga, when the physical world is destroyed by flood and fire. Each destructive period is followed by the succession of creation (srishti), evolution or preservation (sthiti) and dissolution (laya).
A summary of the periods in the cosmic cycles:
1 mahayuga = 4,320,000 years (four yugas) 71 mahayugas = 1 manvantara or manu (we are in the 28th mahayuga) 14 manvantaras = 1 kalpa or day of Brahma (we are in the 7th manvantara) 2 kalpas = 1 ahoratra or day and night of Brahma 360 ahoratras = 1 year of Brahma 100 Brahma years = 309,173,760,000,000 years (one "lifetime" of Brahma, or the universe).