My sump contains heaters, skimmer, carbon, kalk tube, temp probe etc.
If the sump (return pump) is turned off:
-carbon,kalk,probes...stop but ok for a few hours.
-no heater.....water cools to RT..liveable, ONCE.
-Skimmer and overflow contribute to most of the oxygenation of the water....sooooo.. How long can you hold your breath?
In all seriousness, it depends on whats in it and its purpose....But generally 24/7
