Every person is different. The frequency depends on an individual's condition and objectives.
As far as how long a person should see a chiropractor, the answer is simple. As long as the body's spinal structure and surrounding structure go out of place or experience imbalances. These are subjects that a good doctor will educate his patient on. As I have said many times to my own patients, the best thing a physician can do for his patient is to educate them. Educate them on how their body works and what it needs to maintain optimum health. During the course of treatment it gradually becomes more productive for the patient to determine, (through connection to their own body), the course of treatment frequency. These determinations can be made based on the improvements they have experienced and the continued positive effects on the body, mind and Spirit.