My dogs aren't "big" but we still get the largest bag (I'm not sure on weight.. probably slightly under 40 pounds). I store the kibble in airtight containers and leave it in the freezer. A big bag lasts a little over two months I think.

I, however, do believe in feeding the more natural stuff to my boys (human quality kibble and raw). Kaedyn going from acting like a senior to a youngster again is all the proof I need.