Hindus have a cyclic understanding of external history/internal spirituality. The Cycle or "Kalpa", lasting 4,320 million years in the terms of orthodox Hindus, illustrates the pattern of decline in the state of nature and civilization between periods of timelessness when Brahma (Creator aspect of mind/spirit) regenerates the world of existence/reality. There are four yugs or ages in this process from completely pure to completely impure. The final is Kali Yuga or the Iron Age where civilization becomes spiritually degraded, human lives are shortened by violence and disease and there is a general state of decay in nature. This is the worst period before complete destruction which is then followed by a Golden Age.
Hindu traditional prophecies, as described in the Puranas and several other texts, say that the world shall fall into chaos and degradation. There will then be a rapid influx of perversity, greed and conflict, and this state has been described as:
"Yada Yada Hi Dharmasya Glanir Bhavati Bharata, Abhyuthanam Adharmasya Tadatmanam Srijami Aham". Bhagavad Gita (Chapter IV-7)
"Whenever there is decay of righteousness O! Bharatha And a rise of unrighteousness then I manifest Myself!"
Thus whenever there is intolerable evil and chaos in the world, there is an appearance of an avatar. In the current yuga, known as the Kali (the most evil) yuga, "The Lord shall manifest Himself as the Kalki Avatar... He will establish righteousness upon the earth and the minds of the people will become as pure as crystal."
In Hinduism, there is no eternal damnation of souls. There is no end times as well. After this evil Kali yuga ends, the next yuga or epoch would be Satya yuga where everyone will be righteous, followed by Treta yuga,Dwapara yuga and then another Kali Yuga. Thus time is cyclical and the epochs keep repeating infinitely. However, the extent of tolerable evil and degradation in each epoch is different and therefore the threshold that is necessary for the manifestation of God's incarnation is different for each yuga. The current yuga is the most evil and so the threshold for the appearance of the avatar is so high that the world needs to degrade to the maximum levels.