On rides less than 2 hrs I think one bottle of Gatorade (or your drink of choice) and one bottle of water is enough. I freeze them the night before and they'll stay cool up to about an hour. But it is really hot here this time of year (upper 90's). In this heat I've found a Camelbak to be too hot...the sweat from my back can't evaporate. But when it is a little cooler or when I need to carry more water and there are no sources of water along the way then a Camelbak is a necessity.