Time Span of the four Yugas

Please note that 1 Earth year = 360 day nights and not 365 or 366. This is because in scriptures of Hinduism, one year equals 360 day nights.

There are 4 yugs. Their durations in Earth years are as given below:-
Satya yug(also called Krit yug) - 1,728,000
Treta yug - 1,296,000
Dwapar yug - 864,000
Kali yug - 432,000
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Total - 4,320,000

A total of 43,20,000 years constitute a Maha Yuga!

One revolution of the four yugs is called one Maha yug or one Divya yug. So, one Maha yug equals 4,32,000 Earth years.

One day of Brahma equals 1000 Maha yugs i.e. 4.32 billion Earth years. His one night is of the same duration.

I would also like to make a few more interesting points here :
1. Kaliyug is the scale for measurement of one yug. (dwa - twice, tri - thrice and kr- means four times of Kaliyug. In our cycle Kali yuga started the day Lord Krishna left for spiritual world from this material world. It was around 5100 years back, and it takes 4,26,900 more years for kaliyuga to complete. At the end of the kali yuga, when there is no dharma, Kalki will come and start destruction and establish dharma.

2. One interesting fact that I would like to mention here is that in each yug depending upon its duration Vishnu has taken that many avatars. Although, there are lots of avatars which are also known as Vishnu avatar but there are mainly 10.

The first FOUR Avatars happened in Kritha Yuga – Matsya, Koorma, Varaaha and Nrisimha
The next THREE Avatars happened in Treta Yuga – Vaaman, Parasuraama and Sri Rama
The next TWO Avatars happened in Dwapara Yuga – Balaraama and Krishna
The next ONE Avatar will happen in Kali Yuga – Kalki

Also, it is very important to note that the time span of 4 yugs are not cyclic. They are reverse cyclic as per many beliefs. i.e After Kaliyug again Dwapar will come and then Treta and Kritha.
3. With yugas the quality of life and perfection also increases.Another interesting aspect I learnt is on the average life span of the creatures that lived in different yugas and the average dimensions of them!
Safely assuming the average life span of human is around 50 years (across the earth) as of today. Going by the same symmetric math:
Average life span of creatures in Kali Yuga – 50-100 years;
Average life span of creatures in Dwapara Yuga – 100-200 years; (Twice)
Average life span of creatures in Tretha Yuga – 150-300 years; (Thrice)
Average life span of creatures in Krita Yuga – 200-400 years; (Four times)

Assuming the average dimension of human today is around 5.7 - 6ft / 60 – 70 Kgs;
Average dimension of creatures in Kali Yuga – 6 ft / 60 Kgs;
Average dimension of creatures in Dwapara Yuga – 12 ft / 120 Kgs;
Average dimension of creatures in Tretha Yuga – 18 ft / 180 Kgs;
Average dimension of creatures in Krita Yuga – 24 ft / 240 Kgs;

The current start of genetic engineering and other discoveries about life is just a starting point. There is long way to go.

5. Brahma, is not a human being. Brahma has been personified as a human being so that we human beings can understand.

One maha yuga comprises of 43,20,000 years. 1000 maha yugas are equivalent to the day of the Brahma. Brahma's night is also of the same time. Brahma's one day is equivalent to 864,00,00,000 years. Brahma's one second is equivalent to 1,00,000 years on earth. Brahma lives like this for 100 years, and the entire life time of Brahma is equivalent to one breath of Sri Maha Vishnu. When Sri Maha Vishnu exhilarates, the entire world is created, and when he annihilates the entire world is destroyed. This is very close to concept of Big Bang in modern science. The time span between his exhilaration and annihilation is equivalent to 100 years for Brahma.