Ultimately this depends on what your lifestyle, schedule and budget will allow. You should come up with a schedule that realistically fits in to what your personal needs are. Everyone uses their bodies differently and therefore needs a different level of care.
If you are dealing with chronic pain issues, are an avid athlete, or have high levels of stress in your life then weekly or bi-weekly sessions are most ideal. If you are in good health, experience very little pain and are just seeking wellness maintenance then every two to four weeks would be fine.
The important thing is to set up a regular schedule, whether that means every two weeks or every two months for you. Also, get in the habit of scheduling ahead this way you are less likely to brush of your self-care. It is a real treat to look at your schedule for the week and realize that you have a massage scheduled already!