This depends on a couple factors, including what you hope to accomplish, your injury/trauma/accident/surgery/childbirth history, the quality of your diet/nutrition and water intake, how much stress you’re under at work or at home, your overall physical activity (type, duration, intensity, and frequency of physical activity), your other daily activities at home and at work (sitting, computer, phone work, long-term standing, heavy lifting, etc), and even a few genetic factors.
For the average person under chronic stress and a sedentary lifestyle, a massage every 1-2 weeks is best. Athletes who cross-train, lift heavy weights, or train for endurance (i.e. marathons) should ideally get a massage once a week. Moderately active people under less stress who drink plenty of water and sleep well can get by with a massage every 4-8 weeks (or 1-2 months). People with physical injury in their past (through car accidents, abuse, work injuries, or serious sports injuries) should work with their chiropractic doctor and massage therapist to develop an individual plan that includes more frequent sessions at first that gradually taper off into occasional maintenance visits as the symptoms and muscle balance improves.