There's no simple answer to this question. How long treatment takes depends on many factors:
1. The type of problem;
2. The density of the jawbone;
3. The treatment method.
Additional factors:
1. A metabolic condition in the patient;
2. Pregnancy;
3. The patient's motivation and involvement in the treatment (are they wearing their elastics?);
4. The number of missed check-ups, loose brackets and lost aligners.
Generally speaking, mild cases are resolved faster with aligners. The average length of treatment with braces is about 20 months.
This answer is for general information. Your own case needs an examination by an orthodontist.