"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.
OK. So a partial indulgence does lessen the purgatorial suffering, but it cannot be measured in time units, except by indirection.