Goatrider's hit it on the head: pee clear, you are hydrated. Pee yellow, top off. Can't pee, got a headache, cramps, etc, you're in trouble. Next step is passing out and dying.

The body can process as much water as you feed it. However, with too much water you wash essential electrolytes out of your system; I've had many a demented infantryman come to my aid station because they were made to drink 2 quarts of water an hour in the summer and not given time to eat. Not saying you might not need that much...but you MUST eat. Normal food contains everything you need. Salt by itself is not acceptable.

"Normal" consumption rates for water when active range anywhere from 1/2 liter to 2 liters per hour depending on the temperature, humidity, and wind conditions. I can post more on this if anyone's interested.