The easy answer is “whenever you feel you need it.”
For many, this is once a week. For others this means once a month or once a season. Tune into your body and listen to it. Do you think a massage will help you calm down, relax and recover form you lifestyle? If yes, come on in.
Frequency is wholly related to the lifestyle you lead, your age and your activity level. If you are hitting the gym 3 times a week, best to come in every two weeks. If you are traveling a lot with work, got 3 kids hanging on you neck daily and can’t seem to sleep too well . . . maybe you should be coming in a bit more often.
Listen to your body. You will find the answer there.