Hindus believe in four repeating cycles we are currently in the last Kali Yuga which is characterised by lack of virtue but and at the end of this period Kali will judge everyone destroying the wicked and saving the good, starting a new golden age. The other 3 eras are known as Satya Yuga, Treta Yuga and Dvapara Yuga.
Satya Yuga is the era of truth or Golden age where people are closest to the Gods and are governed by them, human lifespan is around 4000 years. In Treta Yuga people start to move away from this ideal slightly and even further in Dvapara Yuga, human lifespan lowerd to 1000 years before entering Kali Yuga.
With this gradual movement away from religion during the four Yuga’s so human lifespan decreases, disease, hunger, war and the breakdown of society become more and more prevelent until Satya yuga come round again.
Each Yuga lasts for many thousands of years Satya Yuga lasts 1,728,000 human years (4,800 god years), the Treta Yuga lasts 1,296,000 human years (3,600 god years), the Dwapara Yuga lasts 864,000 human years (2,400 god years), and finally, the Kali Yuga lasts 432,000 years (1,800 god years).
The decline is gradual each Yuga blending into the next rather than a sudden change. Except from Kali Yuga to Satya Yuga. Every thousand such cycles the earth is also destroyed at the end of Kali Yuga and and then renewed.