Here's some links of interest:
Complete Guide to Triathlon Competition
Daily Peloton - Heat Stress Resolution
From what I gather, under most athletic conditions, you'd need about 500-750ml (16-25oz) of water for complete replenishment from sweating. Is it even possible to sweat more? I guess it depends upon ambient air temperatures and how much moisture you're recovering through your nose.
Maximum absorption-rate looks to be about 900-1000ml/hr (30-34oz) and can put you in danger of hyponatremia if you drink that much. Note that very little absorption of water occurs in the stomach, it's all in the intestines. As long as osmolality of carbohydrates is less than 6-8%, the solution will empty from the stomach at this maximum rate; much faster than necessary. Since your body can absorb water much faster than you'll sweat away (1.5-2x), you'll want to pace yourself in drinking the 500-750ml (16-25oz) of water per hour.
I carry two 24oz bottles of water on my bike and typically go through both in about 2.5 hours (about 20oz/hr). Then I stop off and fill up at gas-stations, beach, etc. Like ericgu said, everyone's body is different, so maintain a log of your rides with weather conditions, exercise-intensity, miles, time and amount of water drunk and weigh yourself before & after to see if that amount was sufficient for replenishing water. Also read that last link about electrolyte replacements as well. Hyponatremia can be fatal and it's caused by a combination of not enough electrolytes and drinking too much water.
Last edited by DannoXYZ; 09-02-06 at 12:32 AM.