Below is an excellent commentary on the condition of being free from attachment to all sin required for gaining a Plenary Indulgence. Fr. Z posted a link to this commentary on WDTPRS a while back:
Plenary Indulgences Not Impossiblehttp://the-hermeneutic-of-continuity.bl ... sible.html
Detachment from venial sin
The most problematic condition is:
[...] the complete exclusion of any attachment to any sin, even venial,
This is not a new provision in the reform of Paul VI. Lepicier in his book Indulgences, their origin, nature and development reported a controversy which was widely current in his own time. Some theologians considered that the actual gaining a plenary indulgence was very rare.
[...] whilst with regard to plenary Indulgences, they teach us in a dogmatical tone that exceedingly few are those who can gain it, and fewer still are those who actually do gain it - perhaps a holy nun in some remote corner of the world, or some saintly hermit dead to this life and its concupiscences. (page 341)
In countering this severe view of indulgences, Lepicier observed that falling into venial sin is not the same as having an affection for venial sin:
From the first no man, however holy, excepting Christ, and His Blessed Mother, can call himself free; but many should be, and in reality are, free from the second. How can we imagine faithful souls, that are anxious to please God, and daily seal this desire with the Bread of Life - and their generation, thank God, is not extinct - how can we imagine such as these to be wilfully attached to that which, though not causing eternal death, yet is infinitely injurious to the Divine Majesty? (page 343)
If there is any doubt about the more lenient view of "detachment from venial sin," it is perhaps worth noting that this view was expressed in 1895 by a Roman professor of theology.
More recently, in the grant of an indulgence for the Year of the Eucharist, the Apostolic Penitentiary restated the conditions for gaining a plenary indulgence. However, when speaking of special conditions for those who are infirm, the official English translation reads:
[...] as long as they are totally free from any desire to relapse into sin, as has been stated above.
We may treat the more "lenient" view as common teaching since the Church clearly intends to grant plenary indulgences that can be obtained by the faithful every day. It would not seem reasonable to do this if it were almost impossible to gain them in practice.
We may therefore encourage people to carry out the works prescribed for the gaining of a plenary indulgence (including, for many, a return to the sacrament of confession) without discouraging them by the rigorist opinion that a plenary indulgence can scarcely ever be gained in fact. It is also a good thing to pray before doing the indulgenced work, asking God to take away all affection for venial sin and conceiving in our hearts a hatred of any sin since all sins displease God who loves us so much.