I only use one when they are showing signs of going into shed. I never keep a humidity box in at all times. Now some species may need higher humidity around the clock, just depends on what species of snakes your talking about, and humidity conditions in your area. Most of my North American kings, ratsnakes & corns don't even need a humidity box because the enclosures I keep them in combined with a large water dish maintains a high enough humidity. Now our ball python is a different animal all together. For him, when he starts showing signs of going into shed, I take spagnum moss, soak in water just enough to get it nice and moist, not dripping, and place in one of his hide spots. It may take a week or two of making sure the spagnum moss is moist, but he has an excellent shed every time. Hope that helps,
Steve