How often should I sand vacuuming?

I don’t think it matters for Ich mostly, but ideally you would want to mix your sand around from the bottom of the tank upwards at least twice a month to avoid gas pockets from forming (and the sand turning from whatever color to black which is a pocket of ammonia).

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Sand is best cleaned at least once a month but it depends on the size of the tank and considering you just had a case of ich, I would be doing it weekly for a while.

Weekly will be fine. This would be the best option.