This article is from the Depression FAQ, by Cynthia Frazier with numerous contributions by others.
Many people decide that their antidepressant is not working prematurely. When one starts an antidepressant the hope is for rapid relief from depression. What must be remembered is that for an antidepressant to work, you must be on an adequate dose of the drug for an adequate length of time. A fair trial of any antidepressant is at least two months. Prior to a two month trial the only reason to abandon an antidepressant trial is if the medication is causing severe side effects. With many antidepressants the dose has to be increased at intervals far above the starting dose. Unfortunately, the two-month period mentioned above, refers to two months following the most recent increase in the dose, not the time from starting the particular antidepressant.