As others have already posted, there is no set answer to this due to many variables. Many times, depending on distance, terrain, weather etc, I have humped a 60 lb backpack with no real problems. On the other hand, I have struggled with a pack half the weight in high altitude, steep/thick difficult terrain.

If we ever had the very unlikely need to "bugout" on foot, again depending on distance, terrain, weather, I would on the lookout for the first baby stroller, winter sled etc that will allow more weight to be carried/pushed/pulled and thusly allows much more options to survive with.