I don't know that there is a hard and fast rule; don't know that it's ever been researched formally. However, one of our seasoned members lives by the rule of 64 oz. a day, plus an additional 8 oz. for every 10 pounds you want to lose.
Like many other things in this process, it's probably a case of what works best for you personally, which may take some trial and error to determine. I don't even think "experts" can agree on how much water is good to drink without considering weight loss, and whether or not other fluids should count toward that total.
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days