Full Members may also find the article here
From "Hydrate of Die? Part II":
Due to individual variability of pacing, intensity, sweat rate, environmental conditions, and sporting demands it is difficult to prescribe the exact level of hourly intake needed to maintain athletic performance. Most athletes who drink to satisfy thirst only replace about 50% of their fluid losses and suffer no performance loss because of it. Historically, the average voluntary intake appears to be +/- 500ml per hour, which sounds dire but most start with a fluid reserve, and in events of several hours duration, as long as an Ironman and even some 24-hour and multi-day stage races , with environmental conditions that are not excessively hot or humid, dehydration does not have great negative impact on performance.