Objection 1. It would seem that Penance should not last till the end of life. Because Penance is ordained for the blotting out of sin. Now the penitent receives forgiveness of his sins at once, according to Ezekiel 18:21: "If the wicked do penance for all his sins which he hath committed... he shall live and shall not die." Therefore there is no need for Penance to be further prolonged.
Objection 2. Further, Penance belongs to the state of beginners. But man ought to advance from that state to the state of the proficient, and, from this, on to the state of the perfect. Therefore man need not do Penance till the end of his life.
Objection 3. Further, man is bound to observe the laws of the Church in this as in the other sacraments. But the duration of repentance is fixed by the canons, so that, to wit, for such and such a sin one is bound to do penance for so many years. Therefore it seems that Penance should not be prolonged till the end of life.
On the contrary, Augustine says in his book, De Poenitentia [De vera et falsa Poenitentia, the authorship of which is unknown]: "What remains for us to do, save to sorrow ever in this life? For when sorrow ceases, repentance fails; and if repentance fails, what becomes of pardon?"
I answer that, Penance is twofold, internal and external. Internal penance is that whereby one grieves for a sin one has committed, and this penance should last until the end of life. Because man should always be displeased at having sinned, for if he were to be pleased thereat, he would for this very reason fall into sin and lose the fruit of pardon. Now displeasure causes sorrow in one who is susceptible to sorrow, as man is in this life; but after this life the saints are not susceptible to sorrow, wherefore they will be displeased at, without sorrowing for, their past sins, according to Isaiah 65:16. "The former distresses are forgotten."
External penance is that whereby a man shows external signs of sorrow, confesses his sins verbally to the priest who absolves him, and makes satisfaction for his sins according to the judgment of the priest. Such penance need not last until the end of life, but only for a fixed time according to the measure of the sin.
Reply to Objection 1. True penance not only removes past sins, but also preserves man from future sins. Consequently, although a man receives forgiveness of past sins in the first instant of his true penance, nevertheless he must persevere in his penance, lest he fall again into sin.
Reply to Objection 2. To do penance both internal and external belongs to the state of beginners, of those, to wit, who are making a fresh start from the state of sin. But there is room for internal penance even in the proficient and the perfect, according to Psalm 83:7: "In his heart he hath disposed to ascend by steps, in the vale of tears." Wherefore Paul says (1 Corinthians 15:9): "I... am not worthy to be called an apostle because I persecuted the Church of God."
Reply to Objection 3. These durations of time are fixed for penitents as regards the exercise of external penance.
Source: Summa Theologica (New Advent)