In the days of the Fatima children, a full rosary was considered 15 decades. Children said only 5 decades (I believe the word was “terco”, meaning one-third) and were expected to work up to saying the full 15 as adults. The Fatima children said the 5-decade rosary. So if we want to pray like them then we too can use the 5-decade rosary (And can say it as many times as we want to make 15 or 20 decades).
Regarding traditional indulgences for the Rosary, the
1866 English translation of the Raccolta (the compendium of indulgences for the Church) indicates that the full Rosary is 15 decades, but the indulgences associated with it, including plenaries, are available for saying only “the third part” of it, in other words 5 decades. Furthermore, it notes that at least one of the indulgences for saying “the third part” goes back to 1726.
http://www.liturgialatina.org/raccolta/mary.htm
So I’m not sure how far back you want to go with your “tradition”, but it seems pretty clear that saying 5 decades daily, or several times a week, while meditating on the mysteries, is a practice that the Church has promoted for at least 300 years approximately. (Under the latest Manual of Indulgences, the Church still provides indulgences, including plenaries under certain conditions, for saying five decades.)
You’re always free to say more if you want. I know elderly retired people who say four or five rosaries a day. I work all day for a living and I sometimes want to pray other prayers as well, so one or at most two is generally my limit.
Also, if you want to say the Rosary like St. Bernadette at Lourdes, you can say the six-decade Brigittine rosary because that’s the one she (and Our Lady) used there.