Concerning the duration of Purgatory, the Church simply tells us it is not a place of eternal pain, but will end at last judgment; neither are we informed of the length of time required for the purification of a soul.
According to St. Thomas Aquinas, the soul, to be reunited to her Creator in Heaven, must be in the state of primitive innocence which adorned her when she proceeded from His hand. The image of God must be entirely restored within her, commensurate to the degree of glory awaiting her in Heaven.
From this it is evident that the suffering souls cannot enter Heaven until perfectly cleansed, either by their pains or by the suffrage of the faithful. With the royal Prophet they cry out in plaintive voices: "As the hart panteth after the fountains of water, so my soul panteth after Thee, O God! When shall I come and appear before the face of God?" (Ps. xli 2-3) They suffer until entirely purified, until the last farthing of their debt is discharged. Increased and intensified pain will probably supply the want of time for the souls who shall not have rendered full satisfaction by the last day.
Prayer: O God! the Bestower of forgiveness and the lover of human salvation, we implore Thee, through the intercession of the Blessed Virgin Mary and all Thy saints, grant to the souls of our brethren, relatives, benefactors, and all the faithful departed, the joys of eternal bliss. Through Christ Our Lord. Amen.
Special Intercession: Pray for the souls of those who are most desirous of obtaining help from you.
Lord grant them eternal rest and let perpetual light shine upon them. May they rest in peace. Amen. (Three times.)
Practice: Mortify your curiosity.
Invocation: My Jesus, mercy!