Why did indulgences change?

I just found my first Daily Missal, given to me in 1950. In it, there is a listing of prayers to be said that dealt with the amount of the indulgence, or grace given. All of the different prayers were listed as obtaining certain amounts of time for reciting them. That amount of time was supposed to be subtracted from the total amount of time we would have to spend in Purgatory for our sins. Example: Jesus, Mary, Joseph, pray for us. Time alloted - 7 years Why did this practice cease?

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Recommended Reading:
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Myths about Indulgences**
*]Primer on Indulgences
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