The Church has never definitively defined the exact momment.
The Church HAS on the other hand, definitively said that the Sacrament is Present upon completion of the Eucharisitic Prayer\Anaphora.
The Church has also defined the critical components of a Valid Eucharistic Prayer\Anaphora.
- Epiclesis
- Words of Institution.
(there are cases where one of the this is implicit, if not explicit, like the Epiclesis in the Roman Canon and the Institutional Narrative as in the Anaphora of Adai and Mari )
Therefore, most Theologians hold that the Eucharist is present when the two main elements of the Eucharistic Prayer have been said.
In the Roman Church, that happens when the Words of Institution have been said ( Epiclesis is First, followed by the Institutional Narrative)
In the East, the Epiclesis follows the Institutional Narrative, so it is held that the Eucharist is present after the Epiclesis.
In either case, the Church had made no official declaration on the exact moment, it remains in the realm of speculative theology.
