There is a time lag from the moment pressure and forces are applied until actual tooth movement occurs. The delay in tooth movement is mainly due to how the body removes bone adjacent to the tooth. Bone is many times not removed immediately next to the tooth, but rather a distance away from the root. Therefore it may take the cells a period of 1-2 weeks before bone is removed close to the tooth.
After the initial phase of nearly no tooth movement, the bone then undergoes significant changes, and the tooth begins to move more rapidly. This will usually occur over the next 2-3 weeks, and is also dependant on the amount of force still active on the tooth.
While there are other factors involved in determining the frequency of appointments, the process just described is often the basis for how often appointments are made. Early in treatment, the orthodontist may use a flexible wire that also has a longer activation time. The wire provides a way to keep a gentle force active over a long period of time.
Therefore, appointments could be scheduled anywhere from 4-12 weeks apart. Later in treatment, as the wires become less flexible, the time between appointments may be scheduled 4-8 weeks apart for more frequent activation and to keep the forces active.
The length of orthodontic treatment is also related to how the teeth move. There are many variables that may contribute to variation in the length of treatment, including growth, cooperation, and differences in the speed of tooth movement. However, since there is a biological limit on the rate of movement, many patients that have similar problems with crowding and a bite that is off will tend to have similar treatment times.