Water intoxication. To avoid overdosing on water (water intoxication), it depends on your kidney function. A healthy kidney can excrete about 1 liter per 1 hr. Try to stay under that to avoid intoxication.
Depends. The amount of water you should drink depends on your body size, how active you are, the weather, and your overall health. Check out this handy calculator at: http://nutrition.About.Com/library/blwatercalculator.Htm.