As we saw in Indulgences Part IV : Crime and Punishment, the Catholic Online article Introduction to Indulgences explained that, in the past, certain lengths of time had been associated with particular indulgences. Catholics had believed that an indulgence said to be worth, say, "300 days" actually meant 300 days less time they would have to spend in Purgatory.
Apparently this had been a great misunderstanding because, according to the article -
"there has never been any way for us to measure how much "good time" that represents. All the Church could say, and all it ever did say, was that your temporal punishment would be reduced -- as God saw fit."
Accordingly, to clarify any misunderstandings and to call a halt to any unseemly totting up of indulgences, the rules were changed in 1967.
Now, a partial indulgence does not carry with it any specific time period. I can only consider it fortunate that the multitude of faithful Catholics who went to their graves believing they had amassed thousands of years time off for good behaviour did not live to see their spiritual account books go up in smoke.