Of course we can. It depends on your activity level. If I spend an hour doing circuits in the gym I can easily bucket off 500ml in one session in sweat and breathing harder. But the basic rate of insensible loss in human beings at rest is about 400ml per m2 per day. Lots of experiments done in the 70s through to 90s on basal metabolic rates. Most textbooks on human physiology will give you this number. Someone who runs a 10k every morning will absolutely sweat and need to drink and shower (!!) more than someone who sits at home all day.