Indulgences used to be designated by time value: 100 Days, 500 days, etc. Now, only the terms partial and plenary are used. Why the change?

This change to “partial” or “plenary” occurred in 1968 when the Enchiridion of Indulgences was issued. There are several reasons this was done. 
 
First, the designation of “days” did not originally reference that time in Purgatory could be lessened. The origin likely had more to do with the penitential practices of the early Church, which were often lengthy and somewhat severe. Given this, one could visit the Confessors of the Faith, in jail, or who had once been jailed for the practice of the faith. Given the esteem these confessors of the faith were held in by the Church, such a visit, and the promise to say prayers, often resulted in time being knocked off one’s penance by the Bishop. Where and when this designation of days, weeks and years came to be applied to the souls in purgatory by the faithful is not exactly clear. 
 
The second problem designating a time value to indulgences is that we are not certain that Purgatory runs on an earth clock. How time passes there, or if there is time, or how time here relates to time there, is all uncertain. 
 
The third problem is that the merit of a prayer or action depends not only on the action done, but on the dispositions and state of the soul of those who do them. Exactly how fruitful the saying of a rosary is, may not be something we can simply gauge by assigning a number. 
 
Most prayers are not sacraments, but sacramentals. Even indulgenced acts related to the reception of the sacraments do not pertain to the sacrament itself, but to the fruitfulness of the reception of it, and the application of those fruits to another. Hence, we are not speaking of something that works automatically (ex opere operato), but rather something that depends for its fruitfulness to a large extent on the disposition of the one who does it (ex opere operantis). 
 
Most people did find the old system of days, weeks and years to be helpful at gauging the general fruitfulness of certain acts or prayers. These days, however, the Church seems to prefer to leave matters such as this less clearly specified for all the reasons stated. And while common sense might value the rosary above a brief prayer or aspiration, even here it is sometimes best to leave things up to God who sees not only the appearance but looks into the heart.