Most Catholics recognize the Litany of the Saints. It was said quite movingly during the funeral of Pope John Paul II.
I went to my old Missal that still has the "amount of time of indulgence" indications in order to give you an example of why there was a numeric number of days or years, and what the logic was, using the Litany of the Saints.
If you prayed the Litany of the Saints at Mass on certain specified holy days as part of the Church community of sacred function the indulgence was "ten years."
If you prayed the Litany of the Saints in the community on those days but there is no sacred special function, the indulgence was "seven years."
If you prayed the Litany of the Saints in the community on any of the other days of the year than those mentioned above the indulgence was "five years."
See, whether purgatory is actually measured in years was never the issue, because who knows but God what purgatory is likely to be for an individual soul? But the Litany of the Saints is a very spiritual and very long prayer that draws upon graceful contemplation of God, Jesus Christ, the Holy Spirit and all the saints, and thus has a very purifying and remedial potency. So it is not a magic charm that one calculates and recites like a formula. It is participating in a community prayer whose intention is this purification. This is why there is a gradation of number of years, but always it is in the context of a community at Mass praying for purification of themselves and those departed (including one's own eventual requirements of purification in purgatory). As the Bible repeatedly demonstrates, when a community gathers to worship God and to pray, and have intentions, prayer is efficacious. But rather than praying for someone who is sick, or for world peace, in these specific prayers the community is praying for purification of their soul both in life and when they die and desire to be in the presence of God. So it is the community, the context of the holy day or ordinary day of Mass or holy function, and the intention of the prayer that creates a dialogue with God that is focused during that prayer on the remediation of sin and the purification of the soul.
I hope this helps in understanding an actual example. So when we all watched the funeral service for Pope John Paul II, people who participated in the Litany of the Saints were dedicating merit of remediation of the stain of sin both for their own behalf and on behalf of the dearly loved departed John Paul II during that specific segment of the holy services. While it is no longer measured in terms of years or days, the Church has an understanding of the intensity and focus of direction of these types of prayers within the context of their intentions.
This is a lot for you all to think about, and it's all good.
And now I'm REALLY sleepy! zzzzzz