Are Indulgences Previously Granted by Popes Still in Effect Unless Specified Otherwise or Later Revoked?

Consider, for example, the prayer to one's Guardian Angel with specified indulgences (extracted from p. 269 of the 1909 book, [*Mary, Help of Christians*](https://ia801604.us.archive.org/20/items/MaryHelpOfChristians/MaryHelpOfChristians_text.pdf) : I know that presently, the Church does not specify partial indulgences anymore in terms of days; nevertheless, I would like to know: QUESTION: If when one sees promulgated a particular partial or plenary indulgence by a Pope, is it still valid [unless specified otherwise (*i.e,* intended to be of short duration) or revoked by a later Pope?] Remark: I believe that in the case of the above prayer to one's Guardian Angel, the indulgence is still in effect for I recall having seen it in the most recent (1994 or so) *Enchiridion Indulgentiarum* although with a different formula for the prayer. Nevertheless, there have been other indulgences granted by Popes in the past (*e.g.,* Prayer to St. Joseph the Worker by Pope St. Pius X with a 500 indulgence attached to it)---that I do not recall having seen in the Enchiridion. Thank you.