By “full rosary” do you mean 5 decades (one set of mysteries) or 15-20 decades (three or four sets)?
If you mean the higher number:
In the
The Secret of the Rosary, St. Louis de Montfort recommended three different sessions (one for each set that he knew) instead of praying all of the decades straight through. I don’t know why, though I have a few speculations.
He also recommended, for those without the time to pray all of it at once, praying a decade when the opportunity arises, such that the whole is prayed by the end of the day. People have noticed that most people are awake for at least fifteen hours a day, and it’s possible to pray one decade each hour; twenty decades is a bit more work, but many people have enough free time for that, too.
Of course, it’s also possible to pray five decades together (probably cycling through the five “continuous” mysteries on different days) and pray the other ten or fifteen throughout the day. I should try that.
As for five decades a day, which has been more of my usual practice so far than a notably higher number:
I ordinarily follow the same practice as
Rich C describes. I’m not opposed to the Luminous mysteries but am happy to treat them as a suggestion.
Another tweak that some people use (I don’t know how common this practice is) is using the relevant set of mysteries on a feast that matches one of the mysteries. That is, when Christmas falls on a Friday and I’m praying only one set that day, I’ll use the Joyful mysteries because they contain the Nativity.
But as others have pointed out, none of these options is mandated.
A final note:
To obtain the plenary indulgence,
praying five decades “without interruption” is necessary. There are also other conditions, like praying the rosary in a certain place or in a certain group–not to mention the conditions for plenary indulgences in general.
Obviously, though, no one should refrain from praying the rosary out of not being able to pray all five decades at once, just as no one should refrain out of not having access to a church (or oratory) or a suitable prayer group, and just as no one should refrain out of not being able to fulfill all of the general requirements for a plenary indulgence. The praying of the rosary is of much spiritual benefit, with or without a plenary indulgence.
Then I started a 54-Day Rosary Novena (that I didn’t finish) but it got me into being able to say the rosary (in about 30 min or less). I don’t pray it every day but I keep my rosary in a pouch in my purse because sometimes, I go to church for Mass during the week, for Adoration, etc so it’s a good thing to do when there’s downtime. I also took it with me on my last trip and pulled it out when we hit some wicked turbulence! Over time, it definitely gets easier so start with the interactive rosaries and go from there. 
:knight2: