By human calculation, a thousand ages taken together form the duration of Brahmā's one day. And such also is the duration of his night.
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 avatāra, 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, 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 billion 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 the inhabitants of Brahmaloka are subject to death, according to the law of material nature.
(BHAGAVAD-GITA--------8:17--------TRANSLATION AND PURPORT).
He is the plenary portion of the Lord, but because Lord Śiva is in direct touch with material nature, he is not exactly in the same transcendental position as Lord Viṣṇu. The difference is like that between milk and curd. Curd is nothing but milk, and yet it cannot be used in place of milk.
The next incarnations are the Manus. Within one day's duration of the life of Brahmā (which is calculated by our solar year as 4,300,000 x 1,000 years) there are fourteen Manus. Therefore there are 420 Manus in one month of Brahmā and 5,040 Manus in one year of Brahmā. Brahmā lives for one hundred years of his age, and therefore there are 5,040 x 100 or 504,000 Manus in the duration of Brahmā's life. There are innumerable universes, with one Brahmā in each of them, and all of them are created and annihilated during the breathing time of the puruṣa. Therefore one can simply imagine how many millions of Manus there are during one breath of the puruṣa.
After 4,300,000,000 solar years, when Brahmā awoke to create again by the will of the Lord, all the ṛṣis like Marīci, Aṅgirā, Atri and so on were created from the transcendental body of the Lord, and I also appeared along with them.
The duration of a day in the life of Brahmā is 4,320,000,000 solar years. This is stated also in the Bhagavad-gītā. And for this same period Brahmājī rests at night in yoga-nidrā within the body of the Garbhodakaśāyī Viṣṇu, the generator of Brahmā. Thus after the sleeping period of Brahmā, when there is again creation by the will of the Lord through the agency of Brahmā, all the great ṛṣis again appear from different parts of the transcendental body, and Nārada also appears. This means that Nārada appears in the same transcendental body, just as a man awakes from sleep in the same body. Śrī Nārada is eternally free to move in all parts of the transcendental and material creations of the Almighty. He appears and disappears in his own transcendental body, which is without distinction of body and soul, unlike conditioned beings.
(SRIMAD BHAGAVATAM------1:6:30-------TRANSLATION AND PURPORT).
There are two types of dissolution of the manifested cosmos. At the end of every 4,320,000,000 solar years, when Brahmā, the lord of one particular universe, goes to sleep, there is one annihilation. And at the end of Lord Brahmā's life, which takes place at the end of Brahmā's one hundred years of age, in our calculation at the end of 8,640,000,000 x 30 x 12 x 100 solar years, there is complete annihilation of the entire universe, and in both the periods both the material energy called the mahat-tattva and the marginal energy called jīva-tattva merge in the person of the Supreme Lord. The living beings remain asleep within the body of the Lord until there is another creation of the material world, and that is the way of the creation, maintenance and annihilation of the material manifestation.
Every living being lives for one hundred years in terms of the times in different planets for different entities. These one hundred years of life are not equal in every case. The longest duration of one hundred years belongs to Brahmā, but although the life of Brahmā is very long, it expires in the course of time. Brahmā is also afraid of his death, and thus he performs devotional service to the Lord, just to release himself from the clutches of illusory energy. Animals, of course, have no sense of responsibility, but even humans, who have developed a sense of responsibility, while away their valuable time without engaging in devotional service to the Lord; they live merrily, unafraid of impending death. This is the madness of human society. The madman has no responsibility in life. Similarly, a human being who does not develop a sense of responsibility before he dies is no better than the madman who tries to enjoy material life very happily without concern for the future.
There are two kinds of dissolutions. One dissolution takes place at the end of the life of Brahmā. At that time all the planetary systems, including the heavenly systems, are dissolved in water and enter into the body of Garbhodakaśāyī Viṣṇu, who lies on the Garbhodaka Ocean on the bed of serpents, called Śeṣa. In the other dissolution, which occurs at the end of Brahmā's day, all the lower planetary systems are destroyed. When Lord Brahmā rises after his night, these lower planetary systems are again created. The statement in Bhagavad-gītā that persons who worship the demigods have lost their intelligence is confirmed in this verse. These less intelligent persons do not know that even if they are promoted to the heavenly planets, at the time of dissolution they themselves, the demigods and all their planets will be annihilated. They have no information that eternal, blissful life can be attained.