For the Plenary indulgence you would indeed lose the indulgence because it’s a condition of gaining it that five decades be said in one sitting! From the
Enchiridion of Indulgences:
"The gaining of the plenary indulgence is regulated by the following norms:
- The recitation of a third part only of the Rosary suffices; but the five decades must be recited continuously. … "
It’s like the indulgence for venerating (kissing) the Cross on Good Friday - you couldn’t gain that indulgence by doing the act on any other day, because it’s part of the conditions that it be done on Good Friday.
Ah, I was thinking of a different set of indulgences, given to members of the Confraternity of the Holy Rosary (more info at www.rosary-center.org):
ROSARY INDULGENCES
For members of the Rosary Confraternity, a plenary indulgence, under the usual conditions, is granted:
on the day of enrollment. (When application is made, a certificate of membership is sent, indicating the day of the enrollment.)
on the following feast days: Christmas, Easter, Annunciation, Purification, Assumption, Our Lady of the Rosary, and Immaculate Conception.
For those who pray the Rosary, a plenary indulgence is granted under the usual conditions, when the Rosary is prayed in Church, or in a Public Oratory, in a family (family Rosary), Religious Community, or Pious Association. Otherwise a partial indulgence is granted.
This is an explanation of indulgences from the same website:
INDULGENCES IN GENERAL
An indulgence is the cancellation of temporal punishment due for sin, when the sin’s guilt has already been pardoned.
An indulgence is partial if it frees the Christian partially from the temporal punishment due for his sins, plenary if it frees him wholly.
Both partial and plenary indulgences can always be applied to the dead, but only by way of suffrage.
Since the Apostolic Constitution of Pope Paul VI on Indulgences, a partial indulgence is no longer expressed in reference to time, i.e. days or years.
A plenary indulgence can be gained only once a day, except by those on the threshold of death.
To gain a plenary indulgence the person must perform the indulgenced act, and satisfy these conditions: Sacramental Confession, Holy Communion, prayer for the Pope’s intention, and freedom from all attachment to sin, even venial sin. If this detachment is not present, or if any of the above conditions are not fulfilled, the indulgence is partial.