I have a metric century loop that I've done many times. In cooler months, I might drink 1.5 water bottles. One day last August-- when the afternoon high peaked at 106º, I drank nine 25oz bottles, and was still 7lbs lighter when I got back home. On very hot days, never stop drinking, and supplement that water-- electrolyte tablets, Gatorade, salt tabs, whatever it takes. If you just drink nothing but straight water and sweat that much, you can blow your electrolyte balance and end up heat exhausted anyway. My doctor told me to have one out of three bottles have electrolyte-- sweating +50oz an hour can get dangerous.

Mind you, I think I have it easier being someplace very, very dry-- on days of 100+ temps, we will occasionally see single digit humidity. The sweat dries really fast, and I get really thirsty. High humidity, I don't know how I would survive.