In the early Church, and for serious sins, one’s assigned penance could often take days, weeks, months, or even years to fulfill. So, for example, a person could be assigned the penance of saying one Our Father every morning for 120 days.
Well, as indulgences developed, they were assigned a certain amount of days by the Church, and these days were a shortening of the penance required to atone for past sins.
So an indulgence of 120 days, for example, would be equal to saying a particular prayer–like the Our Father–every day for 120 days. The merit one could acquire through the latter act, one could acquire through the indulgence, and because of the Church’s use of the Keys.
Today, however, the Church no longer assigns “days” to indulgences, and considers indulgences to simply be either plenary or partial.
If you’d like to know more about this, see this article:
ewtn.com/library/ANSWERS/PRIMINDU.htm