Anywhere from none to 5 litres

It depends where and what I'm doing. If I'm hiking normally, and I expect to have water to get every so often (such as in my typical haunts in the canadian rockies) I will carry about a litre of water, and stop and refill periodically. Usually this looks like I will drink during my hike, stop, chug down the remaining half litre into me, fill the bag with a litre and treat it, and keep going. If I expect to be somewhere where there is no water I will carry a little more, say 2 litres on a climb and traverse of a long dry ridge. If I know I'm going to stop and camp somewhere with no water, I pull out my extra platypus bag, tank them both up, and pack 5 litres in to camp. If it's hotter than hades I'll pack more, etc.

So the answer is "it depends" - but most of the time a litre. sometimes 2.