What happens to the fruits of the Mass applied to a particular soul who is no longer in purgatory?
The general belief is that the duration of a soul's stay in purgatory is determined by how fast the process of purification gets accomplished. Prayers of the people on earth, including Mass Intentions, speed up the process of purification. Here is where one tends to underestimate the benevolence of Gracious God, who keeps an account of all the prayers offered on behalf of each soul. Imagine this situation: Mrs. Smith dies on 28 Nov 2023 and is assigned to purgatory till 29 Dec 2023. Mr. Smith goes to the parish priest and requests him to offer Mass for his wife's soul on every 28th day of the calendar, for a full year to come. The Lord knows this and gives 'credit points' to Mrs Smith so that the benefit of Mass to be said beyond 29 December accrues to her in advance. Of course, the parish priest keeps on saying Mass for the soul of Mrs. Smith, not knowing that his prayers have already been heard!
And it shall come to pass, that before they call, I will hear; as they are yet speaking, I will hear.