I have a variety of packs, from the Large Alice, a Large Kelty, to daypacks and buttpacks. Finding room for the stuff is not a problem.

But how much is too much, weight wise. I'm not the 22 y/o who carred the 100 lb+ radio ruck in Alaska anymore. I have tried to pare the packs into manageable sizes/weights so that if we have to bug out on foot - the final option as far as I am concerned - each pack would be under 30 lbs w/o water, about 35 with. This would give us about a gallon of water apiece, 4 days food, materials to build shelter/fire, extra clothes and socks, mess gear, water purification, and a toilet kit. There is some dupication, but that's for the good.

What would your BOB max weight ideally be if you were on foot?

A lot depends on the terrain and the time of year, so I guess to live up here for a week or so the pack would weigh in around 40-45lbs. (25lbs of gear, shelter and clothing, 12-15lbs of food and 3-5lbs of LPG fuel) A large survival knife or axe wouldn't be of much use!

Interesting question, but rather general. How far do I have to walk, how much water is available, what season?

My SAR pack, available 24/7, in all seasons, could support me for at least 72 hours. One of my compatriots put it on a scale and it weighed 41 pounds, with about two liters of water. I always tweaked the contents depending on the season - in the summer in the desert, it essentially became a giant water bottle. The winter pack was actually lighter. In a true BOB, as opposed to a SAR pack, I would not include as much first aid gear nor as much climbing gear, but the contents would weigh about the same.

Now I am a bit older, and I would try to lighten my pack to somewhere in the 30-35 pound range, a goal that is feasible with some of the newer fabrics and gear that is available. I can easily handle that load indefinitely, and heavier weights for short periods.

The key question is the amount of water you must carry. Nothing else comes close in terms of weight.

You should take a look at your packs. Rucsack design has moved on a lot since the ALICE packs came out in the early 70s. A pack that deep, is carrying gear a long way from your back. That is leverage working against you.
If you know where you are bugging out to: you could dump stuff (food and water are cheap) then you only need enough to get to it. You can dump a few days food and water at every point of the compass for very little. In fact nothing. You use the food as you rotate it.
qjs

That leverage works with you,when you use the Designated Frame-LC-1 or LC-2!Without the frame & A Loaded pack-It will Flat out hurt you,In a short amount of time! Medium or Large ALICE pack!

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.

I agree that the bug out scenario is pretty unlikely, some special situations excepted (hurricanes, perhaps). In most situations, I am going to hang out at home, where most of my resources are concentrated. If I have to leave and automobile travel is not feasible (a distinct possibility) I would start out on a bicycle, which will provide about three or four times the distance for the same effort. If I have to abandon the bike and walk, things are really getting tough.

I am most likely to walk in order to get back home, and unite with my family.

A quick answer that is easy to apply because the weights are not absolutes but proportion of the person's weight:
Never, in anything less than an emergency, or lack of alternatives, and even then only for short distances, pack more than half your weight. It is possible to carry more, up to slightly over 100% of your weight, but you aren't going far or fast and at 50% to 100% the chances of injury go up.

(In most cases you will move more cargo faster with two trips at 50% than one trip at 100%. And you are far less likely to damage your back, knees, ankles.)

A third your weight is about the practical upper limit for most people to carry a pack any significant distance.

Anything more than a quarter your weight is tiresome.

A tenth your weight is about the upper limit of relatively unburdened backpacking where you won't be significantly slowed or tired.

My idea BOB weight if I were on foot would be zero. Put it on a pack horse, a llama, a push bicycle or a 20 yr old companion with lots of brawn and not too many brains.

In my prime I hiked with 50lb thru the woods with no trail at altitude. It was exhausting.
In my prime, after I learned to reduce weight, hiking with 35# was much, much better.

Nowadays..... I can carry 25lb on hiking trails for 15 miles or so but I pay a price for it. A second day would be much harder.

Mostly it depends on your fitness. Thumbrules have assumptions that might not be valid for you.

I think I work with some people for whom a cup of coffee would be too much if they had to leave the building.....

My BOB is pretty heavy, but it lives in my car. If I'm not moving far on foot, I might be glad to have much of the gear that's in it. If I do have to hoof it a significant distance, my plan is to ditch or cache gear appropriately based on the distance, terrain, weather and overall situation.