Commuting I don't think I've ever gone over 20 pounds or so. It's tough to say because I don't know how much library books weigh. Hauling books back and forth is an occupational hazard. As for normal commuting stuff--clothes, food and the like--it's certainly under 10 pounds.

Grocery shopping is another situation entirely. It's not unusual to have 10 lbs each of potatoes, onions and rice, then 5 lbs each of apples, oranges and bananas. That's 45 right there before other produce, meats, canned goods, frozen, and whatever else will fit in the panniers. If I had to guess, my max was about 60.

My rack is rated for 40 kg, 88 lbs. I don't know the weight rating of my Arkel Shopper grocery panniers. I haven't reached it yet, though.