There's no absolute rule; you just need to run it long enough to do the job.
With a DE filter, if your pool is lightly loaded (few swimmers, little leaf debris), you should be able maintain it with *2* 3hour intervals, separated by at least 8 hours. It's not a good idea, usually, to leave your pool completely stagnant for 20 hours, which is what happens with a single short run period.
But . . . if you have a SWCG (chlorine generator) or inline feeder, you may need to experiment. Many SWCG's are not large enough to generate enough chlorine for your pool in only 6 hours. If you are not maintaining chlorine in the most POORLY CIRCULATED areas of your pool -- test just BEFORE your pump starts a cycle -- then you'll need to do something to compensate.
Ben
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