I have some medium to large sized paniers and until recently was able to take my bike to the grocer and walk my bike through the store like a two wheeled shopping cart so I could get a really good idea what I could carry.
I figure that on many occasions I would be getting 40 50 pounds of groceries and the paniers would be filled to their max. For anything more I built a utility trailer that will accommodate a few carts full of groceries and and am presently working on a new hitch assembly so I can pull it behind my Phillip's Twenty folder.
I put a dual drive on my Twenty this week so I would have an adequate gear range for commuting and errands as well as doing things like carrying 40-50 pounds of stuff as well as towing my trailer when needed. The low centre of gravity and outright toughness of the Twenty makes it great little worker and a really nice bike to ride.
The bike with my paniers... I found the factory side stand to not be up to the task of supporting the bike when it was loaded and it had a habit of dropping down where I would then end up hitting it with the crank arm. I removed that stand and installed an old school centre stand which works much much better.
I was packing a good deal of weight when I took this picture... the bags were filled with my usual kit, extra clothes, and bike parts.