By human calculation, a thousand ages taken together is the duration of Brahmā’s one day. And such also is the duration of his night.
…This age of Kali is not at all suitable for self-realization as was Satya-yuga, the golden age, or Tretā- or Dvāpara-yugas, the silver and copper ages. For self-realization, the people in Satya-yuga, living a lifetime of a hundred thousand years, were able to perform prolonged meditation. And in Tretā-yuga, when the duration of life was ten thousand years, self-realization was attained by performance of great sacrifice. And in the Dvāpara-yuga, when the duration of life was one thousand years, self-realization was attained by worship of the Lord. But in the Kali-yuga, the maximum duration of life being one hundred years only and that combined with various difficulties, the recommended process of self-realization is that of hearing and chanting of the holy name, fame, and pastimes of the Lord.
The sages of Naimiṣāraṇya began this process in a place meant specifically for the devotees of the Lord. They prepared themselves to hear the pastimes of the Lord over a period of one thousand years. By the example of these sages one should learn that regular hearing and recitation of the Bhāgavatam is the only way for self-realization. Other attempts are simply a waste of time, for they do not give any tangible results. Lord Śrī Caitanya Mahāprabhu preached this system of Bhāgavata-dharma, and He recommended that all those who were born in India should take the responsibility of broadcasting the messages of Lord Śrī Kṛṣṇa, primarily the message of Bhagavad-gītā. And when one is well established in the teachings of Bhagavad-gītā, he can take up the study of Śrīmad-Bhāgavatam for further enlightenment in self-realization.
Further information on the Age of Kali
Srimad Bhagavatam
By His Divine Grace A.C. Bhaktivedanta Swami Prabhupada
Canto One, Chapter One, Text 21
kalim āgatam ājñāya
kṣetre ’smin vaiṣṇave vayam
āsīnā dīrgha-satreṇa
kathāyāṁ sakṣaṇā hareḥ
kalim—the age of Kali (iron age of quarrel); āgatam—having arrived; ājñāya—knowing this; kṣetre—in this tract of land; asmin—in this; vaiṣṇave—specially meant for the devotee of the Lord; vayam—we; āsīnāḥ—seated; dīrgha—prolonged; satreṇa—for performance of sacrifices; kathāyām—in the words of; sa-kṣaṇāḥ—with time at our disposal; hareḥ—of the Personality of Godhead.
TRANSLATION
Knowing well that the age of Kali has already begun, we are assembled here in this holy place to hear at great length the transcendental message of Godhead and in this way perform sacrifice.
PURPORT
This age of Kali is not at all suitable for self-realization as was Satya-yuga, the golden age, or Tretā- or Dvāpara-yugas, the silver and copper ages. For self-realization, the people in Satya-yuga, living a lifetime of a hundred thousand years, were able to perform prolonged meditation. And in Tretā-yuga, when the duration of life was ten thousand years, self-realization was attained by performance of great sacrifice. And in the Dvāpara-yuga, when the duration of life was one thousand years, self-realization was attained by worship of the Lord. But in the Kali-yuga, the maximum duration of life being one hundred years only and that combined with various difficulties, the recommended process of self-realization is that of hearing and chanting of the holy name, fame, and pastimes of the Lord. The sages of Naimiṣāraṇya began this process in a place meant specifically for the devotees of the Lord. They prepared themselves to hear the pastimes of the Lord over a period of one thousand years. By the example of these sages one should learn that regular hearing and recitation of the Bhāgavatam is the only way for self-realization. Other attempts are simply a waste of time, for they do not give any tangible results. Lord Śrī Caitanya Mahāprabhu preached this system of Bhāgavata-dharma, and He recommended that all those who were born in India should take the responsibility of broadcasting the messages of Lord Śrī Kṛṣṇa, primarily the message of Bhagavad-gītā. And when one is well established in the teachings of Bhagavad-gītā, he can take up the study of Śrīmad-Bhāgavatam for further enlightenment in self-realization.
Bhagavad-gita As It Is 1972 Edition
By His Divine Grace A.C. Bhaktivedanta Swami Prabhupada
From Chapter 4, Text 1
…“In the beginning of the Tretā-yuga [millennium] this science of the relationship with the Supreme was delivered by Vivasvān to Manu. Manu, being the father of mankind, gave it to his son Mahārāja Ikṣvāku, the King of this earth planet and forefather of the Raghu dynasty in which Lord Rāmacandra appeared. Therefore, Bhagavad-gītā existed in the human society from the time of Mahārāja Ikṣvāku.”
At the present moment we have just passed through five thousand years of the Kali-yuga, which lasts 432,000 years. Before this there was Dvāpara-yuga (800,000 years), and before that there was Tretā-yuga (1,200,000 years). Thus, some 2,005,000 years ago, Manu spoke the Bhagavad-gītā to his disciple and son Mahārāja lkṣvāku, the King of this planet earth. The age of the current Manu is calculated to last some 305,300,000 years, of which 120,400,000 have passed. Accepting that before the birth of Manu, the Gītā was spoken by the Lord to His disciple, the sun-god Vivasvān, a rough estimate is that the Gītā was spoken at least 120,400,000 years ago; and in human society it has been extant for two million years. It was respoken by the Lord again to Arjuna about five thousand years ago. That is the rough estimate of the history of the Gīta, according to the Gītā itself and according to the version of the speaker, Lord Śrī Kṛṣṇa.
Also from Chapter 8, Text17
sahasra-yuga-paryantam
ahar yad brahmaṇo viduḥ
rātriṁ yuga-sahasrāntāṁ
te ’ho-rātra-vido janāḥ
sahasra—thousand; yuga—millenniums; prayantam—including; ahaḥ—day; yat—that; brahmaṇaḥ—of Brahmā; viduḥ—know it; rātrim—night; yuga—millenniums; sahasra-antām—similarly, at the end of one thousand; te—that; ahaḥ-rātra—day and night; vidaḥ—understand; janāḥ—people.
TRANSLATION
By human calculation, a thousand ages taken together is the duration of Brahmā’s one day. And such also is the duration of his night.
PURPORT
The duration of the material universe is limited. It is manifested in cycles of kalpas. A kalpa is a day of Brahmā, and one day of Brahmā consists of a thousand cycles of four yugas or ages: Satya, Tretā, Dvāpara, 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 Tretā-yuga vice is introduced, and this yuga lasts 1,296,000 years. In the Dvāpara-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 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 Brahmā, the creator god, and the same number comprise one night. Brahmā lives one hundred of such “years” and then dies. These “hundred years” by earth calculations total to 311 trillion and 40 million earth years. By these calculations the life of Brahmā 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 Brahmās rising and disappearing like bubbles in the Atlantic. Brahmā and his creation are all part of the material universe, and therefore they are in constant flux.
In the material universe not even Brahmā is free from the process of birth, old age, disease and death. Brahmā, however, is directly engaged in the service of the Supreme Lord in the management of this universe—therefore he at once attains liberation. Elevated sannyāsīs are promoted to Brahmā’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 Brahmā and all inhabitants of Brahmaloka are subject to death, according to the law of material nature.
Text pasted from; Causless Mercy
The Hare Krishna Movement 