A 20 oz. bottle an hour during winter, spring, summer, or fall. I have one bottle that holds 28 oz and all the others will hold 24 oz. I wear a Camelback if I know I'll be riding for over 2 1/2 hours. While on a 100 mile ride I got dehydrated and both calves cramped, AT THE SAME TIME. Never again!! I always drink a little more than I need and bring plenty of GU if I ride for more than an hour, usually a pack every 30 minutes or so. I drink GU2O when on rides of over an hour cause it seems to give me a little kick. This may be excessive but having a built in Dietician/Nutritionist in the family (my sister) helps a lot.