Duration of treatment depends on many factors. Usually, for people who had only 1 episode of illness, with a low risk of relapse medicines are given for 6-9 months after symptoms have resolved. If you had multiple episodes of illness in past or have high risk of relapse then longer treatment may be given. Some patients even require lifelong treatment. There is no harm in long term treatment as long as your illness doesn't recur. Kindly consult your psychiatrist to know how long you need treatment. I hope you are consulting a psychiatrist for depression and not a general physician. Also, you must understand that there is no cure for psychiatric illnesses. Medications are meant to control symptoms. Risk of relapse is always there.