The length of time an orthodontic treatment takes depends on the individual patient. An average comprehensive treatment takes around 18 months, but cases vary widely, and sometimes a limited solution can be reached more quickly than a comprehensive one. The length of an orthodontic treatment will depend largely on the extent of the misalignment of the teeth and bite as well as many other factors such as skeletal jaw discrepancies, periodontal disease, missing teeth, excessive tooth wear, TMJ/TMD problems, etc.
A wide variety of issues may be responsible for orthodontic problems. Some of these include thumb sucking, breathing through the mouth, and traumatic injuries. The most important factor, however, is genetics, so you can thank Mom and Dad for your crooked teeth!