The days/weeks/etc. designations are not in the current manual (enchiridion) of indulgences. Here is something that might help. You know how they have those matching donation plans in real life? Where what you do it matched by someone else? Currently partial indulgences are explained in a similar manner. Say it is a prayer you do. This is a helpful action all on its own, but it is “matched” as well. They have a more updated version available than the one I quote below, but this should provide the idea:
The faithful, who at least with contrite heart perform an action to which a partial indulgence is attached, obtain, in addition to the remission of temporal punishment acquired by the action itself, an equal remission of punishment through the intervention of the Church.
Someone let me know if this has changed in the last few years. I don’t have an updated copy yet!