Backpacker magazine did a lot of tests on this. Expert packers on treadmills with electrodes etc. Their conclusion, for full enjoyment, never more than 30 % of normal body weight. I think they based that on heart rate climb after 30 % and things like that which would lower the efficiency and would require more stops. My normal weight should be 180 lbs. Therefore I can carry 54 lbs. Except I actually weigh around 200 right now so add that extra 20 lbs to my pack weight. That means I have to keep the pack weight under 34 lbs. And for me, 34 lbs is just fine. Sure, I can load up and carry 80 lbs if I have to but it sure ain't fun anymore. And many a time I have (with help getting him up there) thrown a mule deer mature buck on my shoulders weighing more than I did and carted him a couple miles to the truck. But it is not fun, just something that had to be done.
I also think that walking speed is even more important. Even with a light pack, if I try to keep up with the fast boys, I'm going to bonk. At my own pace, I can go all day and pass them up but if I try to force a little more speed, the end comes quick.