The rosary is a personal private devotion and can be prayed any way you like to pray it. One decade at a time, one decade a day, you can pray the suggested mysteries on the day suggested, or you can make up your own mysteries. You can start off with the usual prayers or you can add to them or take away from them, and same with the ending, you can do it the way you wish to do it.
Now, if you are praying in a church, or with a group of people, it’s different and I’d say mostly they do stick to the days suggested for the traditional 20 decades. But at home and privately it’s up to you how you do it.
I think for most indulgences the following is prescribed:
**For those who pray the Rosary, a plenary indulgence is granted under the usual conditions, when the Rosary is prayed in Church, or in a Public Oratory, in a family (family Rosary), Religious Community, or Pious Association. Otherwise a partial indulgence is granted. **
That above came from The Rosary Confraternity’s web site.