If it takes more than 5 hours to drain water from the top tank, it is likely that the ceramic dome is slightly clogged with ceramic dust or impurities.
Remove the dome from the top tank and soak it in COOL clean water for around 30 minutes. (Do not use warm or hot water!) Then scrub the surface of the filter on all sides with a soft brush. Rinse the dome and re-assemble to top tank.