In the USA, the most referenced article on fluid replacement (before, during, after) is the position stand from the American College of Sports Medicine.
http://tinyurl.com/23gvbq5 (pdf file, 196 KB)
There are numerous websites that reference that information, admittedly aimed more at runners, but still good information. In that article is the recommendation to replace fluids at the rate of sweating, for optimal performance (avoiding >2% body weight loss from water deficit).
"The goal of drinking during exercise is to prevent excessive dehydration (>2% BW loss from water deficit) and excessive changes in electrolyte balance to avert compromised exercise performance. The amount and rate of fluid replacement depends upon the individual sweating rate, exercise duration, and opportunities to drink. Individuals should periodically drink (as opportunities allow) during exercise, if it is expected they will become excessively dehydrated. Care should be taken in determining fluid replacement rates, particularly in prolonged exercise lasting greater than 3 h. The longer the exercise duration the greater the cumulative effects of slight mismatches between fluid needs and replacement, which can excessive dehydration or dilutional hyponatremia." (typo, word left out, in last sentence I think)
"It is difficult to recommend a specific fluid and electrolyte replacement schedule because of different exercise tasks (metabolic requirements, duration, clothing, equipment), weather conditions, and other factors (e.g., genetic predisposition, heat acclimatization and training status) influencing a person`s sweating rate and sweat electrolyte concentrations."
BOTTOM LINE: NEVER TELL ANYONE HOW THEY SHOULD HYDRATE THEMSELVES.