I agree with Sean that this is a fascinating topic, and also I have always presumed (and still do) that the soul departs the body at the very moment of death (at the cessation of a detectable natural heart beat or brain activity). For what purpose would God have it linger? I was curious if the Church has authoritatively spoken (the emphasis being on “authoritatively” rather than “speculatively”) but that was going to involve more research than I have time for at the moment.
Holy Scripture though gives us some clues I sincerely believe.
St. Matthew 27: 50-51
And Jesus again crying with a loud voice, yielded up the ghost. And behold the veil of the temple was rent in two from the top even to the bottom: and the earth quaked and the rocks were rent.
Fr. Haydock’s commentary on verse 50:
With a loud voice. In this our Redeemer confirms what he had said to Pilate; I have the power to lay down my life, and I have the power to take it up again: for he cried with a loud voice, and at the very hour of the evening sacrifice, to shew that it was by the effect of his own will that he died.
St. Mark 15: 37-39
And Jesus, having cried out with a loud voice, gave up the ghost. And the veil of the temple was rent in two, from the top to the bottom. And the centurion who stood over against him, seeing that crying out in this manner he had given up the ghost. said: Indeed this man was the son of God.
Fr. Haydock’s commentary on verse 39:
The centurion considered the crying out of our Saviour as an effect not of human, but divine power, since it generally happens that people at the moment the soul quits the body are reduced to so debilitated a state, that they are scarce able to utter the least word.
St. Luke 23: 42-43
And he said to Jesus: Lord, remember me when thou shalt come into thy kingdom. And Jesus said to him: Amen I say to thee: This day thou shalt be with me in paradise.
Fr. Haydock’s commentary on verse 43:
I say to thee: This day thou shalt be with me in Paradise; i.e. in a place of rest with the souls of the just. The construction is not, I say to thee this day, &c., but, thou shalt be with me this day in the paradise. Wi.—In paradise. That is, in the happy state of rest, joy and peace everlasting. Christ was pleased by a special privilege, to reward the faith and confession of the penitent thief with a full discharge of all his sins, both as to the guilt and punishment, and to introduce him, immediately after death, into the happy society of the saints, whose limbo (that is, the place of their confinement) was now made a paradise by our Lord’s going thither. Ch.—The soul of the good thief was that same day with Jesus Christ, in the felicity of the saints, in Abraham’s bosom, or in heaven, where Jesus was always present by his divinity. S. Aug.—S. Cyril, of Jerusalem, says he entered heaven before all the patriarchs and prophets. S. Chrys. thinks that paradise was immediately open to him, and that he entered heaven the first mankind.
St. Luke 23: 46
And Jesus crying with a loud voice, said: Father, into thy hands I commend my spirit. And saying this, he gave up the ghost.
[All of these verses seem to me to indicate that the soul leaves the body at the very instant of physical death as we understand it.]