I felt this topic is important enough to make its own thread rather than just within the Bartolo Longo thread.

According to Longo's narration of Our Lady of Pompei to Fortuna Agrelli:

La Madonna stessa aveva suggerito il modo che voleva essere pregata da chi avesse maggior bisogno di suo soccorso, cioè facendo tre Novene per impetrar le grazie, e recitando il Rosario intero di quindici poste; ed altre tre Novene con l’aggiunta del Rosario intero per ringraziamento dopo ricevuta la grazia.

Our Lady herself had indicated how she wished to be prayed to by those most in need of her help, namely by making three novenas to obtain graces, and by reciting the entire Rosary of fifteen decades; and three further novenas with the addition of the whole Rosary in thanksgiving after receiving the favor.
...
… incominciò a fare la prima Novena: poi ne intraprese la seconda, e poi anche la terza, recitando ogni giorno il Rosario intero delle quindici decadi, conforme l’indicazione data dalla Vergine alla signorina Agrelli.

“… she began the first novena; then undertook the second, and then the third, reciting each day the entire Rosary of fifteen decades, according to the indication given by the Virgin to Miss Agrelli.
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Chiunque vuol grazie da me… faccia TRE NOVENE con la recitazione delle 15 poste del Rosario, e TRE NOVENE di ringraziamento.

Whoever wants graces from me… let him make THREE novenas with the recitation of the 15 decades of the Rosary, and THREE novenas in thanksgiving.

Longo here assserts that Our Lady explicitly told Fortuna Aggrelli that the 54-day novena to which she had attached the promises requires the Full Fifteen Decades each day of the 54 days.

Thank you for setting the record straight!

I can't tell you how often I see the "5 Decades" all over the interwebs.

I can't tell you how often I see the "5 Decades" all over the interwebs.

That's because for a hundred years that's what people have been seeing in print.
In 1924 Cardinal Hayes attached the Imprimatur to the booklet printed by Benzinger which showed the novena being prayed with five decades daily.  Burnes & Oates in Dublin also printed it that way the same year.
I wonder how many people would have prayed it if they'd known it should be 15.
For anyone wanting a bookmark for their novena book I offer the following.

Somewhere on the web I saw that when someone begins the Novena, they begin with the Joyful, the next day Sorrowful, and of course the next day Glorious mysteries; and the sequence must be kept regardless of the mysteries that would normally be offered for, say Fridays ( Sorrowful).

The Official Sanctuary site also says 5 decades, in case you ask.
 +W always encouraged 15 decades prayed DAILY.RIP+
Luminous mysteries don't belong, ..good for the Sanctuary.

Longo is the closest thing to a primary source, and he cleary says 15 Decades per day for the 54 Days, and attributes the mention of the 15 to Our Lady, but I keep seeing these sites claiming that it has to be 54 of the 5-Decade Rosary (since after some time, the 5-decade became so popular as to become synonymous with "the Rosary", that they undoubtedly just assumed that, and then Sister Lucy clarified that for the First Saturday Our Lady only requested 5 ... so it probably got conflated).  This rotation of J-S-G-J-S-G was certainly contrived after the fact by people trying to figure out how to to this continuously for 54 days.

In other words, the point of this thread is to debunk all those interwebs sites claiming 5-Decades, as there's ZERO support for the 5-Decades having been requested by Our Lady to Fortuna Agrelli, and every reason to accept Longo's account that she specifically asked for 15.