There seems to be a wide spectrum of people's (as in long distance cyclists') needs. Some sweat a lot and need more than others. Some are saltier sweaters so need more electrolytes (as well as real food or just more salted crisps/nuts). For long randonnees/rides I carry 2 x 500ml bottles (because my frame bag precludes larger ones). I think I drank about 2 litres in the first 306k of PBP (to Fougeres) and was content and comfortable with that intake. Others drink much more, but - I appreciate this is on the low end of the spectrum - on a 'normal', daytime 300 (eg the first half of the Bryan Chapman) in 13/14 hours I'd expect to get through 3 litres or so (either from bottles or drinks at stops). If one's waterworks are still demanding the odd stop, I think that's an indication that hydration balance is in a good place. My constitution can cope (enjoy) milk and buying 2 pints at a stop allows the consumption of one + and the decanting into bottle 2 of the rest for consumption in the next 50+k (say).
My daughter, riding a 90k leg of a triathlon plans on consuming 2 litres (and another litre on the half marathon run), but she's averaging 35+kph/250+w.
