Answered by Frederick J. Frese III, Ph.D.:
Schizoaffective disorder has a rather complex set of defining elements. It is a condition where one experiences symptoms of schizophrenia such as delusions, hallucinations, disorganized speech or behavior, or negative symptoms (diminished energy, will, etc.), while, during the same time period, one experiences a major depressive, manic, or mixed episode. Further, to meet criteria, one must have a continuous disorder of at least one month and have at least a two week period where one evidences the schizophrenic symptoms without the mood component. Regardless of how long one has the condition, mood symptoms must be present for a substantial portion of the total duration of the illness.
In contrast, schizophrenia is diagnosed if these mood aspects are not present, there is a major disturbance in ability to work, in interpersonal relations or in self care, and if the condition continues for more than six months.
As a practical matter one can be diagnosed with schizoaffective disorder having had symptoms for as little as one month. A person will not be diagnosed with schizophrenia until one has been experiencing symptoms for at least six months. Also, in practice many psychiatrists and psychologists tend to consider schizoaffective disorder as a form of schizophrenia. Others, however, tend to view schizoaffective disorder as being more closely related to bipolar or other major affective disorders.