The misting does practically nothing.
I would not plan on watering twice a day unless the sun is very strong and nights are pretty warm. In pumous I would sprinkle real well but slowly so that the pumous gets a chance to soak up water.
Actually, if I had something in nothing but pumous I would sprinkle something organic on it, like coffee grinds, just a little, to improve water holding capability. Then, after it is all washed down into the soil, repeat. Or, ground up pine needles or bark.