Assuming the pack is properly loaded and balanced, and properly secured to the body, about 1/3 of your body weight is going to an average limit for cross country travel on moderate terrain. Your leg joints can't take much more than that without risking damage, either tendon inflammation, or bursa irritation could likely develop. There are variables to consider, but if you are in good health now, your joints are okay, and you have a good pack, I would say 50 lbs would be about as far as you'd want to push yourself.