Original: In his book, The Holy Science, Sri Yukteswar explained that the descending phase of Satya Yuga lasts 4800 years, the Treta Yuga 3600 years, Dwapara Yuga 2400 years, and the Kali Yuga 1200 years.
Simplified: Hindus believe that there are periods of time called yugas ("ages" or "eras"): Satya Yuga, Treta Yuga, Dvapara Yuga, and Kali Yuga.