Six to eight weeks is generally enough time to figure out if you will be positively responsive to an ssri. Depending on the response, your provider can go from there. Individuals may need a longer period or may stay on it longer if there is reason to think that a dosage adjustment will make a difference. But if there is a negative response or no response at all after eight weeks, it doesn't really make sense to keep on the med unless there are particular circumstances indicating that action should be taken.