Roughly 2 million years back
In the Mahabharata (Shanti Parva 348.51-52) we can trace out the history of the Gita as follows: “In the beginning of the millenium known as Treta-yuga, this science of the relationship with the Supreme was delivered by Vivasvan to Manu. Manu, being the father of mankind, gave it to his son Maharaja Iksvaku, the king of this earth planet and forefather of the Raghu dynasty, in which Lord Ramacandra appeared”.
Therefore, Bhagavad-gita existed in human society from the time of Maharaja Iksvaku. At the present moment we have just passed through five thousand years of the Kali-yuga, which lasts 432,000 years. Before this there was Dvapara-yuga (800,000 years), and before that there was Treta-yuga (1,200,000 years). Thus some 2,005,000 years ago. Manu spoke the Bhagavad-gita to his disciple and son Maharaja Iksvaku, the king of this planet earth. The age of the current Manu is calculated to last some 305,300,000 years, of which 120,400,000 have passed. Accepting that before the birth of Manu, the Gita was spoken by the Lord to His disciple the Sun-god Vivasvan. A rough estimate is that the Gita was spoken at least 120,400,000 years ago; and in human society it has been existent for two million years. It was re-spoken by the Lord again to Arjuna about five thousand years ago. That is the rough estimate of the history of the Gita, according to the Gita itself and according to the version of the speaker, Lord Sri Krishna. It was spoken to the sun-god Vivasvan because he is also a ksatriya and is the father of all ksatriyas who are descendants of the Sun god, or the surya-vamsa ksatriyas. Because Bhagavad-gita is as good as the Vedas, being spoken by the Supreme Personality of Godhead, this knowledge is apauruseya, super-human. Since the Vedic instructions are accepted as they are, without human interpretation, the Gita must therefore be accepted without mundane interpretation. The mundane wranglers may speculate on the Gita in their own ways, but that is not Bhagavad-gita as it is. Therefore, Bhagavad-gita has to be accepted as it is, from the disciplic succession, and it is described herein that the Lord spoke to the sun-god. The sun-god spoke to his son Manu, and Manu spoke to his son Iksvaku.
Source: Gita Mahatmya – Folio Vedabase
