I don't necessarily stop at every water source to refill, so I like to leave camp with about a liter and a half in my pack. I would have drunk close to a liter for breakfast, so I'm pretty well watered when I start. That 1.5 liters usually gets me to lunch or a little farther.

This works well along the AT in the South in spring, when the springs and creeks are running. In late summer, that may not be true. This time of year, it's possible for the springs and creeks to be running pretty well one day, and dry the next -- it all depends on where the last line of thunderstorms hit. If you have a specific itinerary, you can call the local maintaining club, or post a question on an AT-specific forum like Whiteblaze.net.

If you know things are dry, then you have to carry a lot more water.