I carry as little water as possible, as long as I'm familiar with the trustworthy water sources in that area. Each liter of water weighs 2.2 lbs, so it adds up quickly. As
@ppine suggested, I carry water treatment drops and collect water as I go during the warm summer months. During the winter, if you can't find running water, you can always melt ice or snow. Ice is preferable because it contains much less air, which means you get a larger amount of water faster.
Unless I'm in a desert or high desert area, I never carry more than 3 liters. And that's typically when I know I'm going to be traveling along a ridgeline with little chance of finding a stream.