Important factors in determining the duration of an orthodontic treatment (and the orthodontics cost as well) are :
the severity of the case, complexity of the orthodontic problem that requires correction
patient’s age
state of health of the teeth & gums
the width of distance that teeth should be moved
location of teeth to be moved
growth and tissue response to treatment
the cooperation of the patient (he should attend all his appointments on time and take care not to damage appliances)
proper use of retainers after the braces removal
expertise of the orthodontist
treatment plan and technique
You should also know that orthodontic treatment does not always end when the dental braces are removed. After the braces are removed, many orthodontic patients need to wear a removable appliance (retainer) for a while to keep their teeth in place.
During this time, you'll still need to visit the orthodontist regularly. Every patient wears a retainer for a different length of time, but typically they should be used all the day for six months & later for some more time only during the sleeping hours. It is very important to wear this removable appliance so your teeth do not go back to their old position.
It is not always easy for the orthodontist to make an exact prediction of the time that a specific patient will need for treatment. Especially for young patients the pattern on which the jaw bones and teeth grow in relation to each other is very important. Unusual, uneven growth may lead to the need of changing the treatment plan. In this case the orthodontic treatment will last longer than expected.
Treatment time is also longer when oral surgery and recovery is needed before or during the braces treatment.
Treatment time for orthodontic corrections has decreased in the recent years thanks to the development of new positioning techniques and modern materials for orthodontic appliances. But everything comes with a cost, and novel techniques using advanced materials (such as invisalign) are not affordable for everyone.

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Orthodontic problems may require expensive treatments before you get a perfect smile. Get the best dental insurance you can afford, so that you will not have to worry about the cost of a necessary treatment. Avoid dental insurance plans with "hidden" special terms such as UCR or LEAT, that could limit your options to a non desirable treatment. Do not let your dental insurance take the decisions for you. Learn what you have to check before you choose a dental insurance.