I live with Russians, not Hermanns, but much of the care is essentially the same.
I tend to dust my torts food quite infrequently, like once every other week.
My thinking is that I'm already providing them a rich and varied diet so that intentionally adding calcium might end up being too much of a good thing.
To that end, I would say that dusting your tort's food once a week is probably better than 3 times a week.