Well, I see people saying a gallon a day. I personally think that's a little high but of course that depends on the individual really. Also depends on water along the trail. If water isn't too much of a problem along the trail and you've got iodiine tabelts or a filter of some sorts, you could probably make due with carrying less than 2 liters, filling up as needed along the way. Obviously though, if water is scarce along the section you're on, a gallon is probably your safest bet.

Have fun.
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In all things of nature there is something of the marvelous.-Aristotle