Yuga

To Biardeau the linear nature of the declining dharma and the cyclical nature of the yugas make an unhappy marriage, and she ultimately rejects the idea of a true correspondence or rapport between the appearance of Vișņu as avatāra and the yuga cycle.

Source: wiktionary

There is, you will note, a pattern of decreased duration in the yugas. Moving from the Krta Yuga, the yuga closest to primordial perfection, each progressive yuga grows shorter and shorter, as chaos and decrepitude increase in prominence.

Source: wiktionary

To explain further, there are four ages or millenniums called yugas. The duration of Satya-yuga is 1,728,000 years. The duration of Treta-yuga is 1,296,000 years. The duration of Dvapara-yuga is 864,000 years. The duration of Kali-yuga, the present age, is 432,000 years, and began around 5,000 years ago. These four yugas make one cycle, and one thousand cycles equal one day of Brahma, after which there is a partial annihilation of the universe during Brahma's night. Lord Brahma lives for 100 years, 360 days in each year.

Source: wiktionary