Objection 1. It seems that a man is bound to repay a favor at once. For we are bound to restore at once what we owe, unless the term be fixed. Now there is no term prescribed for the repayment of favors, and yet this repayment is a duty, as stated above (Article 3). Therefore a man is bound to repay a favor at once.
Objection 2. Further, a good action would seem to be all the more praiseworthy according as it is done with greater earnestness. Now earnestness seems to make a man do his duty without any delay. Therefore it is apparently more praiseworthy to repay a favor at once.
Objection 3. Further, Seneca says (De Benef. ii) that "it is proper to a benefactor to act freely and quickly." Now repayment ought to equal the favor received. Therefore it should be done at once.
On the contrary, Seneca says (De Benef. iv): "He that hastens to repay, is animated with a sense, not of gratitude but of indebtedness."
I answer that, Just as in conferring a favor two things are to be considered, namely, the affection of the heart and the gift, so also must these things be considered in repaying the favor. As regards the affection of the heart, repayment should be made at once, wherefore Seneca says (De Benef. ii): "Do you wish to repay a favor? Receive it graciously." As regards the gift, one ought to wait until such a time as will be convenient to the benefactor. On fact, if instead of choosing a convenient time, one wished to repay at once, favor for favor, it would not seem to be a virtuous, but a constrained repayment. For, as Seneca observes (De Benef. iv), "he that wishes to repay too soon, is an unwilling debtor, and an unwilling debtor is ungrateful."
Reply to Objection 1. A legal debt must be paid at once, else the equality of justice would not be preserved, if one kept another's property without his consent. But a moral debt depends on the equity of the debtor: and therefore it should be repaid in due time according as the rectitude of virtue demands.
Reply to Objection 2. Earnestness of the will is not virtuous unless it be regulated by reason; wherefore it is not praiseworthy to forestall the proper time through earnestness.
Reply to Objection 3. Favors also should be conferred at a convenient time and one should no longer delay when the convenient time comes; and the same is to be observed in repaying favors.
Source: Summa Theologica (New Advent)