In an arid environment, it's 1 liter/quart an hour for 38C/100F and up, .5 for below that. See FM 3.05-70 Ch 13, section 29 for more details. You can conceivably have to drink up to 4-6 gallons a day during deployments, field exercises, etc. It takes a lot for a healthy person to hurt themselves from over-hydration, assuming they've got enough sodium and such in the diet.

Obviously, a normal person engaging in moderate exercise in normal environments (i.e. work in a climate controlled office and get around 2 hours of jogging a week) should be drinking about half their body weight in pounds in ounces of water a day, plus another cup for each alcohol drink, and depending on their caffeine addiction level maybe extra water for coffee and soda (caffeine addict's bodies do not treat caffeine as a diuretic, everyone else does) or about 2-4 liters a water a day. The .5 or 1.0 liter of water an hour is advisable for anyone in the normal background during periods of heightened activity, .5 for normal, 1 for hot.

By the way, for anyone curious, the link below goes to a site that has the Army survival manual on it, it's basically the survivalists handbook, the link goes straight to CH 13 but you should be able to go to any chapter you want. Personally I find it to be more detailed and more user-friendly than most books you can get at the store, many of which use it as a reference anyway Also, for skeptics on hydration, it is based of extensive scientific research, a summary of which with references can be found at:

http://ftp.rta.nato.int/public/PubFullText/RTO/MP/RTO-MP-HFM-086/MP-HFM-086-06.pdf

edit: if the link doesn't work, try: http://www.equipped.org/FM3-05.70SURVIVALMANUAL.pdf

Actually that's a better link anyway, the pictures are in color and I think the one I gave is abridged