A substantial proportion of patients suffering from schizophrenia-spectrum disorders (SSDs) exhibit a general intellectual impairment at illness onset, but the subsequent intellectual course remains unclear. Relationships between accumulated time in psychosis and long-term intellectual functioning are largely uninvestigated, but may identify subgroups with different intellectual trajectories. Eighty-nine first-episode psychosis patients were investigated on IQ at baseline and at 10-years follow-up. Total time in psychosis was defined as two separate variables; Duration of psychosis before start of treatment (i.e. duration of untreated psychosis: DUP), and duration of psychosis after start of treatment (DAT). The sample was divided in three equal groups based on DUP and DAT, respectively. To investigate if diagnosis could separate IQ-trajectories beyond that of psychotic duration, two diagnostic categories were defined: core versus non-core SSDs. No significant change in IQ was found for the total sample. Intellectual course was not related to DUP or stringency of diagnostic category. However, a subgroup with long DAT demonstrated a significant intellectual decline, mainly associated with a weaker performance on test of immediate verbal recall/working memory (WAIS-R Digit Span). This indicates a relationship between accumulated duration of psychosis and long-term intellectual course, irrespective of diagnostic category, in a significant subgroup of patients.