Not only prayer, but one can also give up some of the unpleasantness in one's life (i.e., a toothache, back pain) for them. Or skip a meal once in a while for them.
Offering prayers and suffering for the souls in purgatory is a daily part of my life.
Well, this should be a relatively easy matter. Given that saying the Prayer of St. Gertrude the Great releases 1000 souls from purgatory each time it is said (http://www.catholicspot.com/purgatory.htm), and assuming a cumulative world population of 106,456,367,669 (http://www.prb.org/Content/ContentGroups/PTarticle/0ct-Dec02/How_Many_People_Have_Ever_Lived_on_Earth_.htm), and with a current world population of Catholics of 1 billion, only 1 out of ten Catholics would have to say the prayer once to empty out the place (even assuming everyone that ever lived is in purgatory- obviously not the case).
Alternatively, we could take Jesus at his Word when he said "It is finished".
Why did Christ die?
bookmark.
I'm not religious, just reporting what I saw.
the repentant thief on the cross (and Christ) might argue the point - no time for works, but he was saved none the less