Typically, once every week or two is ideal for keeping your muscle tissue pliable and in good shape, if you can afford it. If you are in chronic pain or have a special issue to address, you might need to come weekly (or even twice a week) until you feel better.
Once you're feeling good, once a month is probably the minimum for maintaining the health of your tissue. If you start stretching the massages out too far, then your muscles can revert to their old patterns and you'll have to start all over again to restore their suppleness and pliancy. Listen to your body, but don't wait too long in an effort to save money.
Make sure you find a good massage therapist that you feel comfortable with and whose style you like. You might even want to have a few different therapists you can call on. They all have their own styles and specialities. Sometimes your body might need a vigorous deep tissue massage and sometimes something gentler, like craniosacraltherapy.