My daughter and I carried 21 days food on a trip in the Wind Rivers about 12 years ago. We ended up catching so many fish that we had tons of food left over. We also were able to "hitch" a ride out with fishermen on a boat rather than walk 12 miles of rattlesnake infested sagebrush so our actual trip was 18 days. I think all we brought less than a liter of white gas - we were able to cook on fires a lot.

However, we did not cover a lot of distance but rather did an almost entirely off-trail route. At one point we went 1 mile in 8 hours - had to build a raft of driftwood to get our packs around a cliff!

So I guess the question is meaningless unless you specify exact trail conditions - elevation gain, weather (weight of other gear needed), off trail conditions, etc.

I have never done a long "all trail" route. Most of my 60% off trail routes I average about 8-9 miles per day and go out for 10-13 days.