general rule of thumb is one large bottle an hour. Personally, I couldn't drink 60 ounces an hour, hour after hour without becoming totally bloated. There is a limit to how much water your body can absorb in a given period of time. And too much water can actually lead to hyponatemia.
This article I thought was somewhat helpful. It suggests 16- 24 ounces an hour.
http://www.dailypeloton.com/heatstress.asp
" REHYDRATION RATE during an event requires between 16-24 fluid ounces liquid each hour. Warning: Athletes who consume 34 fluid ounces per hour or more may predispose themselves to diluting serum sodium stores resulting in hyponatremia, a clinical medical emergency."
Another article of interest:
Exercise intensities exceeding 70% VO2max will progressively suppress gastric emptying. The highest reported rate of gastric emptying is 2400ml/hour, in resting subjects. During exercise gastric emptying rarely exceeds 1-1.2 litres/hour, unless a large volume is maintained in the stomach. Runners in particular experience discomfort with high volumes in the stomach, and in race situations it is unlikely that athletes could drink more than two litres/hour, even if they could tolerate it. Some degree of dehydration is therefore inevitable when rates of fluid loss exceed sustainable drinking capacity.
In practice, competing endurance runners and canoeists generally drink about 500 ml/hour, and dehydrate at a rate of 500-1000 ml/hour. Even triathletes, who as a group are relatively knowledgeable about nutrition and hydration, have been shown to lose an average of 1.7% and 3.7% of bodyweight for a 3-hour and a 12-hour event respectively. Whereas cyclists are known to tolerate 1.2 litres/hour of a 6% CHO-electrolyte drink, runners are more likely to experience discomfort, and may believe that time lost through slowing down to drink may not be recovered. The volume that most athletes choose to drink during exercise replaces less than 50% of their losses. With this in mind, effective post-event hydration is vital in between consecutive heats or events spanning several days.
http://www.pponline.co.uk/encyc/0824.htm
The really scientific approach is drink enough that you keep peeing, and your pee's clear.