Sat Yuga

The Satya Yuga (Sanskrit: सत्य युग), also called Satyug, or Kṛta Yuga (Sanskrit: कृत युग) in Hinduism, is the first of the four Yugas, the "Yuga (Age or Era) of Truth", when humanity is governed by gods, and every manifestation or work is close to the purest ideal and humanity will allow intrinsic goodness to rule supreme. It is sometimes referred to as the "Golden Age". The Satya Yuga lasts 1,728,000 years. The goddess Dharma (depicted in the form of cow), which symbolises morality, stood on all four legs during this period. Later on in the Treta Yuga, it would become three, followed by two in the Dvapara Yuga. Currently, in the immoral age of Kali, it stands on one leg.

The Yugas are said to succeed each other almost endlessly. After the perfect Satya Yuga, a decline marks the Treta Yuga. Further decline brings about the Dwapara Yuga, and after it comes the final and dark Kali Yuga, a time of wickedness, when man kills another man. At the end of the cycle Kalki is said to take birth and reestablish righteousness, thus beginning a new Satya Yuga.

...as described in the Mahabharata, a Hindu epic:

Amongst the four eras, the Satya Yuga is the first and the most significant one. Knowledge, meditation, and penance hold special importance in this era. All the pillars of Dharma are present in totality. During Satya Yuga, all people engage only in good, sublime deeds. Ashrams become devoid of wickedness and deceit. Natyam (such as Bharatanatyam), according to Natya Shastra, did not exist in the Satya Yuga "because it was the time when all people were happy". This Yuga extends up to 1,728,000 years. Lord Vishnu incarnated in four forms i.e. Matsya, Kurma, Varaha and Narsimha in this era.The only text which was considered credible and was followed was Manu’s Dharma Shastra. The average life expectancy of a human being in Satya Yuga was approximately 4000 years.

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Wikipedia