How often?
It really depends on the horse, how quickly it respond to the massage/soft tissue manipulation. Some cases respond immediately and others respond better with follow up treatments (i.e long standing problems).
A regular massage benefits the horse like it does ourselves, think how much better we feel after having a massage.
Not only does it minimize the risk of injuries, few human athletes would compete without preparing their bodies using massage and stretching exercises.
Have you ever played sport/exercised one day and then couldn't move the next day ! Now think how the horse feels, but with you on his back as well.