At least a gallon.
If you don't drink enough water...you'll get dehydrated...symptoms of dehydration include: Dry or sticky mouth, low or no urine output - concentrated urine appears dark yellow, not producing tears, sunken eyes, lethargic or comatose (with severe dehydration)...that's from the National Institutes of Health website...honestly, when I don't drink enough water/fluid, I get really tired and just don't want to do anything. It can seriously effect you because about 70% of your body is made up of fluids...try getting some sugar-free drink mix to put in your water - that may help you chug it down. Just be careful of the artificial sweetners they use...too much of that shit and I get an awful headache.