"The duration of the material universe is limited. It is manifested in cycles of kalpas. A kalpa is a day of Brahma, and one day of Brahma consists of a thousand cycles of four yugas or ages: Satya, Treta, Dvapara and Kali. The cycle of Satya is characterized by virtue, wisdom and religion, there being practically no ignorance and vice, and the yuga lasts 1,728,000 years. In the Treta-yuga vice is introduced, and this yuga lasts 1,296,000 years. In the Dvapara-yuga there is an even greater decline in virtue and religion, vice increasing, and this yuga lasts 864,000 years. And finally in Kali-yuga (the yuga that we have now been experiencing over the past 5,000 years) there is an abundance of strife, ignorance, irreligion and vice, true virtue being practically nonexistent, and this yuga lasts 432,000 years. In Kali-yuga vice increases to such a point that at the termination of the yuga the Supreme Lord Himself appears as the Kalki avatara, vanquishes the demons, saves His devotees, and commences another Satya-yuga. Then the process is set rolling again. These four yugas, rotating a thousand times, comprise one day of Brahma, the creator god, and the same number comprise one night. Brahma lives one hundred of such "years" and then dies. These "hundred years" by earth calculations total to 311 trillion and 40 billion earth years. By these calculations the life of Brahma seems fantastic and interminable, but from the viewpoint of eternity it is as brief as a lightning flash. In the Causal Ocean there are innumerable Brahmas rising and disappearing like bubbles in the Atlantic. Brahma and his creation are all part of the material universe, and therefore they are in constant flux.
In the material universe not even Brahma is free from the process of birth, old age, disease and death. Brahma, however, is directly engaged in the service of the Supreme Lord in the management of the universe--therefore he at once attains liberation. Elevated sannyasis are promoted to Brahma's particular planet, Brahmaloka, which is the highest planet in the material universe and which survives all the heavenly planets in the upper strata of the planetary system, but in due course Brahma and all the inhabitants of Brahmaloka are subject to death, according to the law of material nature."
Bhagavad-gita 8:17
In Satya-yuga, the golden age, the principles of religion work fully. The average lifespan is a hundred thousand years, and self-realization is attained by prolonged meditation.
In Treta-yuga, the silver age, the principles of religion are reduced by a fraction of one fourth. The average lifespan is ten thousand years, and self-realization is attained by performance of great sacrifices (yajnas).
In Dvapara-yuga, the copper age, the principles of religion are reduced to one half. The average lifespan is one thousand years, and self-realization is attained by worship of the Lord in the temple.
In Kali-yuga, the iron age, the principles of religion are reduced to one fourth, and gradually diminish to zero. The average lifespan is one hundred years, and self-realization is attained by hearing and chanting the names of the Lord.
