A partial indulgence commutes only a certain portion of the penalty; and this portion is determined in accordance with the penitential discipline of the early Church. To say that an indulgence of so many days or years is granted means that it cancels an amount of purgatorial punishment equivalent to that which would have been remitted, in the sight of God, by the performance of so many days or years of the ancient canonical penance. Here, evidently, the reckoning makes no claim to absolute exactness; it has only a relative value.God alone knows what penalty remains to be paid and what its precise amount is in severity and duration. Finally, some indulgences are granted in behalf of the living only, while others may be applied in behalf of the souls departed. It should be noted, however, that the application has not the same significance in both cases. The Church in granting an indulgence to the living exercises her jurisdiction; over the dead she has no jurisdiction and therefore makes the indulgence available for them by way of suffrage (per modum suffragii), i.e. she petitions God to accept these works of satisfaction and in consideration thereof to mitigate or shorten the sufferings of the souls in Purgatory.
(Indulgences, scroll to VARIOUS KINDS OF INDULGENCES)
"To say that an indulgence of so many days or years is granted means that it cancels an amount of purgatorial punishment equivalent to that which would have been remitted, in the sight of God, by the performance of so many days or years of the ancient canonical penance."
Let's unpack this slowly as it is exactly what I have proposed.
Let's use that novena card that Grammy used to have on her bedside table. It said down at the bottom INDULGENCE 36 DAYS.
In prayerfully and devoutly saying the Novena, one would do the spiritual equivalent of 36 days of ancient canonical penances (sackcloth and ash).
The equivalence was with the ancient canonical penances, not with any time lessened out of Purgatory.
So Grammy had a choice. She could fast and wear sackcloth and ash for 36 days OR she could say her novena devoutly. No wonder she chose the novena.
As to the length of the purification process for individual souls, nothing can be said in terms of years. Pope Alexander VII, Decree 18 March 1666 in Denziger 1143
See also Fundamentals of Catholic Dogma, L. Ott, pg 485.
Great question.