Objection 1. It would seem that one person is not delivered from this punishment sooner than another. For the more grievous the sin, and the greater the debt, the more severely is it punished in Purgatory. Now there is the same proportion between severer punishment and graver fault, as between lighter punishment and less grievous fault. Therefore one is delivered from this punishment as soon as another.
Objection 2. Further, in point of duration unequal merits receive equal retribution both in heaven and in hell. Therefore seemingly it is the same in Purgatory.
On the contrary, is the comparison of the Apostle, who denotes the differences of venial sins by wood, hay, and stubble. Now it is clear that wood remains longer in the fire than hay and stubble. Therefore one venial sin is punished longer in Purgatory than another.
I answer that, Some venial sins cling more persistently than others, according as the affections are more inclined to them, and more firmly fixed in them. And since that which clings more persistently is more slowly cleansed, it follows that some are tormented in Purgatory longer than others, for as much as their affections were steeped in venial sins.
Reply to Objection 1. Severity of punishment corresponds properly speaking to the amount of guilt: whereas the length corresponds to the firmness with which sin has taken root in its subject. Hence it may happen that one may be delayed longer who is tormented less, and "vice versa."
Reply to Objection 2. Mortal sin which deserves the punishment of hell, and charity which deserves the reward of heaven, will, after this life, be immovably rooted in their subject. Hence as to all there is the same duration in either case. It is otherwise with venial sin which is punished in Purgatory, as stated above (Article 6).
Source: Summa Theologica (New Advent)