The collection of all four yugas is called a Chaturyuga. One day of Lord Brahma consists of 1000 such Chaturyugas. This time is called "Kalpa". During this time 14 Manus come and go. Each Manu is created by Lord Brahma Himself, who assigns the Manu with the task of continuing the process of creation by creating generations of mankind called 'Manvantara'. One Manvantara is approximately 71 Chaturyugas. After the Manvantara, Lord Brahma creates another Manu, to carry on the process of creation and procreation. In this new Manvantara, Lord Vishnu takes on a new form, and a new Indra and new Saptarishis are appointed. The current form of Vishnu is the famous four-armed figure having a discus, a lotus, a conch and a mace in each hand. The current Indra is Purandar. The current Saptarishis are Kashyapa, Atri, Vashista, Vishvamitra, Gautama, Jamadagni and Bharadvaja. Later on I will tabulate the other Saptarishis in all the 14 Manvantaras of this present Day of Brahma or the present Kalpa (1000-Chaturyuga-period).
After 1000 Chaturyugas have passed, Brahma's Day is over, which leads to Brahma's Night. During this time, there is no creation as Lord Brahma takes rest. It continues for the same amount of time as Brahma's Day. After the night is over, Brahma commences a fresh creation. The night of Brahma starts with 'Pralaya' (annihilation) and ends with 'Srishti' (creation/manifestation).
=> The actual duration of a Manvantara is 71*4*1000 years, with some additional years, which sum upto 8,52,000 divine years; or 30,67,20,000 human years.
=> 14 times this number constitues a day of Brahma, as there are 14 Manvantaras in one Brahma's Day.
=> Overall, 4,320,000 years termed as 1 Chaturyuga.
=> 1 day of Brahma constitutes 1000 Chaturyugas (= 4,320,000,000 years).
=> Thus 1 year constitutes 360*4,320,000,000 = 1,555,200,000,000 years. Brahma lives for 100 Brahma years.
=> Hence the lifespan of Brahma is 100 years = 100*1,555,200,000,000 = 155,520,000,000,000 years
The following table will clearly link our time-scale to that of Lord Brahma's -
365*(1/4) human days = 1 Deva Ahoratra (1 day + 1 night)
360 Deva Ahoratras = 1 Deva Vatsara
12,000 Deva Vatsaras = 1 Chaturyuga **
71 Chaturyugas = 1 Manvantara
14 Manvantaras = 1 Kalpa
2 Kalpas = 1 Day of Brahma + 1 Night of Brahma
360 Days of Brahma = 1 Brahma Varsha
Brahma's life span is 100 Brahma Varshas, as we know.