There are several sources of information on Yuga of which the most important are the Puranas, the Jyothisha Siddhantas and Mahabharata.
Hindu texts describe four yugas (world ages) in a Yuga Cycle—Krita (Satya) Yuga, Treta Yuga, Dvapara Yuga, and Kali Yuga
Where, starting in order from the first age, each yuga’s length decreases by one-fourth (25%), giving proportions of 4:3:2:1.
Each yuga is described as having a main period (a.k.a. yuga proper) preceded by its yuga-sandhyā (dawn) and followed by its yuga-sandhyāṃśa (dusk), where each twilight (dawn/dusk) lasts for one-tenth (10%) of its main period.
Lengths are given in divine years (years of the gods), each lasting for 360 solar (human) years.
Each Yuga Cycle lasts for 4,320,000 years (12,000 divine years) with its four yugas and their parts occurring in the following order:
Krita (Satya) Yuga: 1,728,000 (4,800 divine) years
Krita-yuga-sandhya (dawn): 144,000 (400 divine)
Krita-yuga (proper): 1,440,000 (4,000 divine)
Krita-yuga-sandhyamsa (dusk): 144,000 (400 divine)
Treta Yuga: 1,296,000 (3,600 divine) years
Treta-yuga-sandhya (dawn): 108,000 (300 divine)
Treta-yuga (proper): 1,080,000 (3,000 divine)
Treta-yuga-sandhyamsa (dusk): 108,000 (300 divine)
Dvapara Yuga: 864,000 (2,400 divine) years
Dvapara-yuga-sandhya (dawn): 72,000 (200 divine)
Dvapara-yuga (proper): 720,000 (2,000 divine)
Dvapara-yuga-sandhyamsa (dusk): 72,000 (200 divine)
Kali Yuga: 432,000 (1,200 divine) years
Kali-yuga-sandhya (dawn): 36,000 (100 divine)
Kali-yuga (proper): 360,000 (1,000 divine)
Kali-yuga-sandhyamsa (dusk): 36,000 (100 divine)
The current cycle’s four yugas have the following dates based on Kali Yuga, the fourth and present age, starting in 3102 BCE.
There are 71 Yuga Cycles (306,720,000 years) in a manvantara, a period ruled by Manu, who is the progenitor of mankind.
There are 1,000 Yuga Cycles (4,320,000,000 years) in a kalpa, a period that is a day (12-hour day proper) of Brahma, who is the creator of the planets and the first living entities.
There are 14 manvantaras (4,294,080,000 years) in a Kalpa with a remainder of 25,920,000 years assigned to 15 manvantara-sandhyas (junctures), each the length of a Satya Yuga (1,728,000 years).
A kalpa is followed by a pralaya (night or partial dissolution) of equal length forming a full day (24-hour day).
A maha-kalpa (life of Brahma) lasts for 100 360-day years of Brahma, which lasts for 72,000,000 Yuga Cycles (311.04 trillion years) and is followed by a maha-pralaya (full dissolution) of equal length.