As much as you can! You can pray a little part of it throughout the day so at the end of the day it is one full mystery. You can pray it once a week, but it is about doing it as much as you can! It is said that if you pray the Rosary every day that it can lead to peace and our Mother Mary had come and explained the 15 promises of praying the Rosary. However, if at the season of your life, it is nearly impossible, then don't feel guilty about it. Just do your best.