The answer is: not more than 20% of your body weight, and ideally, it'll be about 10% of your body weight. If you weigh 200 lbs, you'd be fine with carrying 40 lbs for a day hike. If you're 160 lbs, you'd be okay with a ~30 lbs pack. These packs are a bit heavy, and you'll definitely feel the weight after 10+ miles.