Objection 1. It would seem that the time for contrition is not the whole of this life. For as we should be sorry for a sin committed, so should we be ashamed of it. But shame for sin does not last all one's life, for Ambrose says (De Poenit. ii) that "he whose sin is forgiven has nothing to be ashamed of." Therefore it seems that neither should contrition last all one's life, since it is sorrow for sin.
Objection 2. Further, it is written (1 John 4:18) that "perfect charity casteth out fear, because fear hath pain." But sorrow also has pain. Therefore the sorrow of contrition cannot remain in the state of perfect charity.
Objection 3. Further, there cannot be any sorrow for the past (since it is, properly speaking, about a present evil) except in so far as something of the past sin remains in the present time. Now, in this life, sometimes one attains to a state in which nothing remains of a past sin, neither disposition, nor guilt, nor any debt of punishment. Therefore there is no need to grieve any more for that sin.
Objection 4. Further, it is written (Romans 8:28) that "to them that love God all things work together unto good," even sins as a gloss declares [Augustine, De Correp. et Grat.]. Therefore there is no need for them to grieve for sin after it has been forgiven.
Objection 5. Further, contrition is a part of Penance, condivided with satisfaction. But there is no need for continual satisfaction. Therefore contrition for sin need not be continual.
On the contrary, Augustine in De Poenitentia [De vera et falsa Poenitentia, work of an unknown author] says that "when sorrow ceases, penance fails, and when penance fails, no pardon remains." Therefore, since it behooves one not to lose the forgiveness which has been granted, it seems that one ought always to grieve for one's sins.
Further, it is written (Sirach 5:5): "Be not without fear about sin forgiven." Therefore man should always grieve, that his sins may be forgiven him.
I answer that, As stated above (Question 3, Article 1), there is a twofold sorrow in contrition: one is in the reason, and is detestation of the sin committed; the other is in the sensitive part, and results from the former: and as regards both, the time for contrition is the whole of the present state of life. For as long as one is a wayfarer, one detests the obstacles which retard or hinder one from reaching the end of the way. Wherefore, since past sin retards the course of our life towards God (because the time which was given to us for the course cannot be recovered), it follows that the state of contrition remains during the whole of this lifetime, as regards the detestation of sin. The same is to be said of the sensible sorrow, which is assumed by the will as a punishment: for since man, by sinning, deserved everlasting punishment, and sinned against the eternal God, the everlasting punishment being commuted into a temporal one, sorrow ought to remain during the whole of man's eternity, i.e. during the whole of the state of this life. For this reason Hugh of St. Victor says [Richard of St. Victor, De Pot. Lig. et Solv. 3,5,13 that "when God absolves a man from eternal guilt and punishment, He binds him with a chain of eternal detestation of sin."
Reply to Objection 1. Shame regards sin only as a disgraceful act; wherefore after sin has been taken away as to its guilt, there is no further motive for shame; but there does remain a motive of sorrow, which is for the guilt, not only as being something disgraceful, but also as having a hurt connected with it.
Reply to Objection 2. Servile fear which charity casts out, is opposed to charity by reason of its servility, because it regards the punishment. But the sorrow of contrition results from charity, as stated above (Question 3, Article 2): wherefore the comparison fails.
Reply to Objection 3. Although, by penance, the sinner returns to his former state of grace and immunity from the debt of punishment, yet he never returns to his former dignity of innocence, and so something always remains from his past sin.
Reply to Objection 4. Just as a man ought not to do evil that good may come of it, so he ought not to rejoice in evil, for the reason that good may perchance come from it through the agency of Divine grace or providence, because his sins did not cause but hindered those goods; rather was it Divine providence that was their cause, and in this man should rejoice, whereas he should grieve for his sins.
Reply to Objection 5. Satisfaction depends on the punishment appointed, which should be enjoined for sins; hence it can come to an end, so that there be no further need of satisfaction. But that punishment is proportionate to sin chiefly on the part of its adherence to a creature whence it derives its finiteness. On the other hand, the sorrow of contrition corresponds to sin on the part of the aversion, whence it derives a certain infinity; wherefore contrition ought to continue always; nor is it unreasonable if that which precedes remains, when that which follows is taken away.
Source: Summa Theologica (New Advent)