For a properly fitted frame pack: no more then 1/4 of you're ideal body weight, unless you are in very good shape, then it's 1/3. IIRC, this is the maximum weight that can be carried for prolonged periods without physical degredation.

For a properly fitted rucksack (sternum strap, no hip belt), I'd put it at 20-25 lbs starting weight. Since a fair amount is food and water, the pack weight will decrease over time.
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