I believe the results of the test are stored in the Archive Manager (DDM) database. So, they will be stored according to the value of the MAX_EVENT_DAYS parameter in the $SPECROOT/SS/DDM/.configrc file. Out of the box default is 45 days.
Not 100% sure if they are stored in the Spectrum Report Manager (SRM) database. If so, then they will be stored according to the retention period setup for that database.
Joe