How long does it take to diagnose schizoaffective disorder?

It depends. This depends on the reliability of the patient, other sources of history (such as medical records documenting past episodes of illness, results from medication trials, etc), and following the person over time -- past the point where mood is stable. If you can't get to that point, you don't know whether psychosis is present without mood episode -- thus it make take longer to firm up the diagnosis.