I use an Eberlestock J34. size depends on if I need to expand it (2,300-4,600 cubic inches). I usually stay out 3-4 days, but this year i'm staying out until I get one.
For backpack hunts where I stay 10-12 days in the wilderness I use a 5500-6000 ci external frame. Need the extra room for food to enable me to stay out that long.
I use a 4500 for 10 days or less... I have to strap the bag, tent and pad to the outside of the pack though, there's no room for extras inside with all the food like BOHNTR said...
My Badlands Sacrifice should arrive tomorrow. I've heard some guys say they can get 5 to even 7 days worth of stuff packed in it (3450 cu) so I'm hoping thats possible. I looked one over in the store and it seems to be do-able. At least 5 days, no problem. 6-7...we'll see. I'm hoping to keep upgrading/lightening my gear so that should help.
I also use an Eberlestock bag for backcountry hunts. I have the Dragonfly and it holds everything I need for up to 8 days and compacts to a nice daypack once camp is set up.
I use a Badlands 2200, and can get 3 days out of it if I pack it right. Any longer than that I usually have someone take the horses in and drop me off, then come back in and pick me up at a pre determined time.
Osprey Aether which is about 4500cu and it works great for 5 days, I strap my pad on the outside but everything else fits good. Picked up the KUIU 6000 this year gonna see how well it works, and see if I can stretch the time line out a little.