Carl Sagan, in his book, Cosmos writes:
"Hinduism is the only religion in which the time scales correspond... to those of modern scientific cosmology. Its cycles run from our ordinary day and night to a day and night of the Brahma, 8.64 billion years long, longer than the age of the Earth or the Sun and about half the time since the Big Bang"
Indeed, the divisions of time propounded by the ancient sages is incredible, and the life span of different classes of living beings as described in the various Puranas appears unbelievable- human life does not even form a tiny part in the affairs of the universe according to this world view.
We may start with the description of the Yugas - there are 4 yugas in the this timescale which repeat themselves cyclically.These 4 Yugas are called Satya, Treta, Dvapara and Kali Yuga - which last for 12,000 years of the demigods.Different classes of living entities dwell in these Yugas on earth according to their level of consciousness. These 4 Yugas are experienced by only those living entities who are in the earthly planetary systems - there is no effect of these Yugas on the demigods
A year of the demigods is 360 years of human beings. Taking this into account, the duration of various Yugas
Satya Yuga - 4,800 x 360 = 1,728,000 years Treta Yuga - 3600 x 360 = 1,296,000 years Dvapara Yuga - 2,400 x 360 = 864,000 years Kali Yuga - 1200 x 360 = 432, 000 years
So, combined these yugas last for 4.32 million or 43.2 lac human years.One cycle of the Yugas is known as a Catur Yuga.
One thousand Catur Yugas combine to form Brahma's day and Brahma's night is of the same period - a night and day of 4.32 billion years! Thus, Brahma's day lasts for 8.64 billion years. Brahma's months also have 30 days, and Brahma thus lives on for 100 years. The lifespan of Brahma comes out to be an astounding 311 trillion 40 billion human years.
Creation of the planetary systems begins at the start of the Brahma's day and these creations continue to exists throughout the fourteen Manus - thus each of these Manu's have a lifetime of 71 catur-yugas, which is nearly 305 million years. The various posts in heaven, such as that of Indra, the seven sages and his followers, simultaneously come in the reign of Manu.
There's an interesting point to note, however, that one's life endures only for hundred years, in terms of lifespans in different planets - which means although Brahma lives for trillions of years according to human beings. he lives only for hundred years according to his perception. Time is relative to the consciousness of the living entity - 100 years of human beings amount to only 3 months of the demigods in the higher planetary systems, and in Brahma's timescale - it does even count up to 1 second!
These figures are astronomical - and as such they might be brushed off as "fictitious" or "mythological". However, we need not be conditioned by our own experiences of lifetimes while judging the correctness of such figures.A hundred year lifespan of human beings does not imply the non existence of living entities who may have such extraordinary life spans.If we go by such logic, then we might end up being the "frog in the well"!