For anyone who is really “into” what Our Lady of Fatima said…
It’s my understanding that while in the English language, “rosary” is used to refer to the 5-decade rosary, the 15-decade rosary, or the 20-decade rosary (expanded with the new mysteries many years after Fatima), the Portuguese language spoken by Our Lady to the children of Fatima uses the word “terco” which specifically refers to the 5 decade rosary. Children back then were taught to pray the 5-decade rosary with the understanding that after developing the habit, they might “graduate” as adults to the 15-decade rosary.
Sometimes Our Lady would tell the children to pray the “terco” every day, which would have been basically doing what good Catholic children were expected to do. At the time the kids were having a hard time doing even that, as Lucia said they would just say the first words of the prayers in order to get done quicker so they could play, and Francesco didn’t enjoy saying it at all before he saw Our Lady.
Other times she told the children they would have to pray “many tercos” and at one point she appeared referring to herself as Our Lady of the Rosary, which in Portuguese referred to the 15-decade rosary. This has been taken by some to mean that Our Lady wanted us to pray the 15-decade rosary.
However, it is reasonable to think that Our Lady would like us to pray as much of the rosary as we reasonably can during a given day. Some days I can pray 15 decades and other days I can only pray 5, or not even. I currently try to get 5 decades a day done and during November I tried to accomplish 20 decades a day but I only managed to meet that goal for about half the days of the month.
The Church, in its Manual of Indulgences, gives the rosary indulgence for 5 decades accompanied by pious meditation on the Mysteries.
The Blue Army of Our Lady of Fatima and other rosary confraternities also typically promote saying the 5-decade rosary once a day, or 3 times a week, etc.
I do know people who say 4 or 5 rosaries a day, but they tend to be older retired folks who have the time. I could also possibly say more rosaries if I removed other prayers from my daily schedule, but I would like to say a variety of prayers and I’m sure Our Lady is not going to be mad if I said a Divine Mercy and the St. Bridget’s 12-year prayer rather than an additional 5 decades of Rosary. All the prayers are going to the same place.
Also, if somebody could only say 1 decade a day, or even just 3 Hail Marys a day, that’s better than nothing. We are supposed to try to “work our way up” by increasing what we do over time, so if you start with one 5-decade Rosary, you can try to make it a better Rosary by concentrating more or meditating better, and then at some point maybe you want to try to do additional decades.
I hope this helps.