I will try to present some evidence for you in reverse order, starting with dates we can both agree on and then working backward. Keep in mind that despite my profession, I am no liturgical effort, and I’m sure others on here can help me out.
First, a couple of assumptions:
I think we can both agree that the idea of the Real Presence in the Eucharist is established by the Reformation, so I will leave any discussion from that era out.
Since the Eastern Orthodox Church also believes in the Real Presence in the Eucharist (and the Roman Catholic Church recognizes the legitimacy of the Real Presence in the Orthodox Eucharist), we can assume that the doctrine predates the Great Schism between the Catholics and Orthodox.
(Now, that said, the Orthodox will not use the word “Transubstantiation”. They leave the miracle of the change of the bread and wine into the Body and Blood of Christ as a mystery, but they DO believe in the Real Presence of Christ, and in basically the same way as Catholics.)
Ok, right away I suggest that we can push the date much further back, since the Oriental Orthodox and the Assyrian Church of the East BOTH believe in the Real Presence in the Eucharist, and they broke off from the Catholic Church as a result of the Council of Ephesus in 431 and the Council of Chalcedon in 451. This fact pretty well speaks for itself, so we now have the date pushed back to the early to mid 5th century. Bear in mind that the word “Transubstantiation” did not develop until much later, as a result of challenges to this doctrine. The Catholic Church does not generally define dogmas unless the particular belief is challenged and requires further explicit explanation. You cannot judge when the belief began by the introduction of the word Transubstantiation.
While we’re at it, it’s probably good to look at what the Council of Ephesus had to say on the matter:
Council of Ephesus**
“We will necessarily add this also. Proclaiming the death, according to the flesh, of the only-begotten Son of God, that is Jesus Christ, confessing his resurrection from the dead, and his ascension into heaven, we offer the unbloody sacrifice in the churches, and so go on to the mystical thanksgivings, and are sanctified, having received his holy flesh and the precious blood of Christ the Savior of us all. And not as common flesh do we receive it; God forbid: nor as of a man sanctified and associated with the Word according to the unity of worth, or as having a divine indwelling, but as truly the life-giving and very flesh of the Word himself. For he is the life according to his nature as God, and when he became united to his flesh, he made it also to be life-giving” (Session 1, Letter of Cyril to Nestorius [A.D. 431]).
As an official pronouncement of the church, the date of the belief is now safely established at least as far back as 431 A.D.