I suspect you'll get a lot of different answers, since so much depends both on gear configuration and your food/cooking lifestyle. Mine is not necessarily a typical example!
My pack has a main body capacity of 2600 cubic inches (42 L). I have no trouble getting 9-10 days of food plus my gear in that space without resorting to the extension collar. The only gear items not in that 42 L are my tent, fishing rod, water bottle, the day's snacks/lunch, a possible pair of wet socks being dried and my rain gear, all of which ride in the outside mesh pockets. Now that I have a cuben fiber tent, I will probably move it into the main pack for protection, although since it's quite small it still won't overflow into the extension collar.
My food is mostly home-dehydrated, with a few freeze-dried items such as most meat and a few veggies (bought in bulk mostly from Packit Gourmet) added in. Many ingredients are bought off the store shelves, of course: breakfast cereal, dried milk, cereal bars, nuts, rice (cooked and dehydrated at home), quinoa (ditto), cous-cous, beans and lentils (both these last cooked and dehydrated at home), some dried fruit (which I dry further in my dehydrator to reduce weight). For longer trips I tend to use more freeze-dried fruit which is a lot lighter. You'll find lots of meal ideas on Sarbar's (Sarah's) website at www.trailcooking.com.
What I did to allow volume/weight for my food when pack hunting was to make up a couple days' actual meals and from that figure out the approximate weight and volume of a week's worth of food. I mocked up the volume by stuffing paper into plastic bags and the weight with several weights I had on hand; this all went into my food bag. I added a liter of water and a large fuel canister and called it good.
The most important space-consumer in your pack is your non-consumable gear--how much space will that take up? Things like a bulky tent, synthetic fill sleeping bag or bulky (when packed) pad can fill up a lot of space!
That's why most people advise buying your pack last, after you have everything you're going to put inside. That way there are no guessing games with either volume or the amount of weight the pack needs to support.