This depends on the goal or reason for the session. Sessions performed with the specific intent of relieving problems should ideally be done weekly.
Variables that predispose a horse to muscle injuries, and therefore more frequent sessions, include: Conformation, overall health, training schedule, show schedule, conditioning, rider influence, injury.
The following is a guideline to recommended session frequency in the “ideal world”:
Leisure riding horse, ridden 3-5 times per week: Every 3-4 weeks.
The Serious Competitor: Every 2 weeks
The Highest Level Competition Horse (e.g. racehorses): Every 1-2 weeks
Retired and Pleasure Horses: Every 6 weeks
Injured and Rehabilitating Horses: Under veterinary supervision, a full session once a week for two weeks, and then sessions further apart as the horse improves.