Even on the harsh deserts of Arrakis, the the water recycling capacity of a stillsuit prevent the wearer from would only lose a thimbleful of water a day. If you figure a thimble holds about 10 milliliters of water, and an astronaut normally consumes 2.7 liters of water per day through eating and drinking, that’s only a loss of .4% of the body’s daily water. Pretty impressive for a desert race with long life but limited resources. (In case this wasn’t clear, I’m talking about George Frank Herbert’s Dune here.)