I normally need to carry 3 to 4 liters of water which weighs about 1lb per liter.
1L of water is about 1kg, which is about 2.2lbs. So 3-4L of water should come in at roughly 6.6-8.8lbs. (Conversions are approximate and ignore the temperature of the water.)
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I tend to pack pretty light, with an ultra-light tent, pad, and sleeping bag. I carry a light knife (Mora), a small saw, and a small shovel. I bring extra clothing, a first aid kit, and a few small odds and ends (small thing of bug spray, small flashlight, etc).
With all this stuff plus food and about 3L of water for a night, my pack comes in at maybe 20 pounds, tops. I'd obviously need some more food for more than an overnight. I can save weight on water by bringing only one bottle and using water purification.
In warmer weather, I travel a little lighter. In cold weather, I travel a little heavier. The stuff above is a kind of average that assumes I need to bring long pants, extra socks, a light fleece, and such. If it's the dead of summer I can cut a lot of that weight by bringing fewer clothes and a light blanket. If it's down near freezing, I add a few more pounds for extra warmth.
As long as I'm not sprinting on the uphills (which I do when I'm bored), I can carry my kit for a while without getting tired. I got used to carrying an unbalanced bag with about 10 pounds of books, netbook, and water to class all day. 20 pounds spread out well in a balanced and adjusted pack isn't much harder to carry.
I suppose I could hike for a bit with 30-40lbs, but I don't think I'd want to.