"Massage is most effective when is it made part of a regular health routine"
Think of all the things that make you healthy: Good diet, appropriate exercise, good hygiene, mental health care, routine medical care. What is something that all of these things have in common? Rarely can any of these things be done one time and the results become permanent. As much as it would be nice to eat one vegetable once a year, or do one single circuit of exercise and magically your body is in perfect shape, it doesn't work that way. We need to constantly be aware of our diets, exercise regularly, bathe and brush our teeth even more routinely in order to keep ourselves healthy. Massage is the same way. Many people will "treat themselves" to a massage once a year or for a "special occasion" - which is great, and better than never getting massage at all. However, for those once-a-year massages only scratch the surface of massage's potential.
Massage can do some pretty amazing stuff in one shot. It can calm down a spasm, immediately release hormones that help lower blood pressure and stress, and loosen tight muscles. But if you get a massage more frequently, those muscles might not tighten up again so quickly. The spasm might not happen as severely, if at all. As for stress, regular massage may keep you from feeling the effects as quickly or you may react to the stressors differently. This is because every time you receive a massage, you are changing how your body works. If you only introduce these changes to your body every so often, the body will see it, acknowledge it, then go back to the way it had been working before. Whereas, if you keep reintroducing the change often, it will start to re-learn how things should be working and functioning, and therefore making each massage session more effective.
It is similar to learning a new skill: very few people can make a perfect golf swing or catch a baseball on the very first try. It takes multiple attempts to teach the muscles, body, and brain how to work together to execute the skill properly, then many more attempts to perfect the skill. Massage is the same. Each massage helps the muscles learn "this tightness isn't ok, this is how you should feel/work". Massage trains the muscles to learn what is good and what isn't, so over time, is something happens that should cause tightness, the muscles are better prepared to deal with it without it getting super bad, super fast - and with regular massage, it is likely that the muscles never get a chance to get that bad to begin with because there is less time to let it build up.
So just like brushing your teeth or getting into jogging, the best way to reap the maximum benefits of getting massage is to make it part of a routine. One of the best ways to get it into a routine is to schedule the next appointment when you check out or before you leave the previous appointment. How often do we say "Oh, I meant to do this but forgot/got distracted/something else came up and it went out of my head"? Your Massage Therapist can best advise you as to what frequency of massage would be good for whatever you are needing, then it is up to you to create and keep that routine to let the massage help you work the best.