Although I am not a Catholic I believe I can answer your question more determinately.
The only place where the exact instant seems to have been conjectured within Roman Catholic tradition is by St. Thomas in the Summa Theologica, Part 3, Question 75, ‘Article 7. Whether this change is wrought instantaneously?
And therefore it must be said that this change, as stated above, is wrought by Christ's words which are spoken by the priest, so that the last instant of pronouncing the words is the first instant in which Christ's body is in the sacrament; and that the substance of the bread is there during the whole preceding time. (St. Thomas in the Summa Theologica, Part 3, Question 75, ‘Article 7. Whether this change is wrought instantaneously?)
I am assuming that when he says ‘the last instant of pronouncing the words’ he means two instantaneous events. The bread turns into his body after saying ‘this is my Body’ and another event when the wine turns into his blood, after saying ‘this is the chalice of my Blood’. Reading the official documents are somewhat complicated because Greek philosophical terms about substances, their relative quantities and the accidents or things we experience from our sense when observing those substances are couched in the terms used to explain the intended doctrine. Basically in laymen terms, our senses perceive ‘accidents’ but only the mind knows the ‘substance’. What this essentially implies is that Catholics while still seeing the accidents of regular bread and wine that the substance of Christ is miraculously maintaining, this transformed substance. For example the transformed material is to literally understood as the physical body of Christ:
Christ and all the constituents of a true body, such as bones and sinews, but also Christ whole and entire. (Catechism of the Council of Trent)
However, since the notion of the Presence in the Mass is somewhat higher than each element it implies not just the body and blood but the soul and divinity of Christ is present, i.e. a complete Christ in both events. In other words, this higher presence of soul and divinity occurs when becoming the bread and then is some sense becomes more completed when the wine is transformed. The notion of a complete unified Presence can be seen from the Creed of Pope Pius IV.
I profess, likewise, that in the Mass there is offered to God a true, proper, and propitiatory sacrifice for the living and the dead; and that in the most holy sacrament of the Eucharist there is truly, really, and substantially, the Body and Blood, together with the soul and divinity, of our Lord Jesus Christ; and that there is made a conversion of the whole substance of the bread into the Body, and of the whole substance of the wine into the Blood, which conversion the Catholic Church calls Transubstantiation. I also confess that under either kind alone Christ is received whole and entire, and a true sacrament. (Creed of Pope Pius IV)
So now the question occurs, if the bread and wine is only the physical literal body of Christ, hidden behind accidents and quantities that present to our eyes a mere substance of bread and wine, how then is the Presence of Christ actually also sitting behind the veil of what our senses see? This answer is tied up in the term ‘concomitance’. The soul and divinity of Christ is understood as being present through another term called ‘concomitance’. This in effect means the bread is all of Christ including his divine presence and so it the blood.
The full development of all these ideas can now be understood by the more difficult terms used by Sty. Thomas where ‘accidents’, ‘quantities’ and ‘concomitance’ bind the whole doctrine:
Nevertheless, since the substance of Christ's body is not really deprived of its dimensive quantity and its other accidents, hence it comes that by reason of real concomitance the whole dimensive quantity of Christ's body and all its other accidents are in this sacrament. (St. Thomas in the Summa Theologica, Part 3, Question 76. The way in which Christ is in this sacrament ,Article 4. Whether the whole dimensive quantity of Christ's body is in this sacrament?)
The reason St. Thomas seems to have desired to determine a precise moment is in handling objections that might indicate truth to the doctrine of consubstantiation, where according to a Lutheran view the substance of the body and blood of Christ are present alongside the substance of the bread and wine. To refute the idea of consubstantiation, transubstantiation must occur instantaneously, otherwise even for 1/1000 of a second part of the bread must still be bread. Therefore, the change must occur so quickly in time that there is no measureable instant of a gradual change. This change must be therefore, according to St. Thomas at the moment the last words of the consecration are spoken by the priest. Whether this moment is at the time the last syllable is uttered or the time that the last syllable is heard among those present would probably be exact for the purposes of St. Thomas. If I had to guess he means when the last syllable is leaving the vocal cords through the mouth of the priest because depending upon the distance of those present each might hear the final sound at slightly different times, voiding the completely instantaneous concept.
Although St. Thomas is quite exact in his identification of when the bread and wine become the blood and body of Christ, he leaves the precise moment of when they return to just bread and wine uncertain. He seems to indicate the whole transformation is outside of normal physics causing regular moments of succession to cease in the transformation. This makes the time where the presence is no longer there not a proper ‘moment’ following another ‘moment’ along the science of time. In other words, the starting moment is identified as it is preceded by another moment, but the end, not being preceded by a proper moment is not exactly determinate. Or in his words:
Of this time no instant is to be taken as proximately preceding the last one, because time is not made up of successive instants, as is proved in Phys. vi. And therefore a first instant can be assigned in which Christ's body is present; but a last instant cannot be assigned in which the substance of bread is there, but a last time can be assigned. (St. Thomas in the Summa Theologica, Part 3, Question 75, ‘Article 7. Whether this change is wrought instantaneously?)