Jimbo,

Is that running just one burner or two at a time?  I run the backpacking stoves (MSR Windpro) and will take 6 of the 8 oz. isobutene/propane canisters for 4-6 people on a 10 day trip and usually come back with one full.  I do more than just boil water for freeze dried stuff (pancakes, red beans and rice, fish fry, fish chowder, eggs) and don't come near the cook times you are doing. 

So, J&M, like Jimbo says, depends on your cooking style, how much stove time you have and if you're running both burners.  I'd venture to say you'd probably do well with 4 - 1 pound bottles for 9 days if you use your fuel judiciously.  If you're perkin' a lot of coffee and making long cook foods, maybe toss in a 5th one.

pd