Hmmm 50-75%, but it depends a lot on what we are making that week. We typically buy a lot of stuff on Sundays (meats, canned items like tomatoes and such, cereal, bread, lunch items, oils, etc) and then I run back to the store during the week for the perishables. Farmer's market day is Thursday so we'll pick up at least some things from there. We always have things like cereals, breads, etc though.
Summers are different than winter, too...less access to fresh fruit and veggies in the winter. With the kid at home during the summer, we often have more crap in the house. Not like junk food, but packaged stuff; we try to buy the least processed packaged foods available. Lots of ingredient listing reading and comparing. If he's going to eat a granola bar, it's going to be a healthier and less processed one. He's never even had a Twinkie, lol.