How much I'm carrying depends on a lot of variables: where I'm going to be hiking, how hot it is. If it's high in the mountains, with frequent water sources, I start with a liter and, when it's half gone, pick up another liter and plunk a Katadyn Micropur tablet in it. By the time the I've drunk other half-liter, the new liter is ready to drink. I will start with at least 2 liters, maybe 3, if I'm ridge-walking or it's a hot day or if I can't expect to replenish water at least once during the day. And that's after "tanking up" before I leave camp! I'm not a desert hiker, but if I did, I'd carry at least a gallon a day.