1.25 lb of peanut butter, cheese, or lard per day should be plenty of calories. That probably isn't what you meant, though.
Long ago a botched resupply left me with six days worth of food for a twelve-day stage. Walking out and hitting a grocery store wasn't an option. It wasn't fun at all.
From my standpoint, food on any trip up to a week is a luxury. But this whole outdoor travel thing has substantial psychological and "morale" elements. It is a pleasure to eat. Actually, when I look at a lot of trail journals they degrade into largely a record of what was eaten and who it was eaten with, or possibly where it was eaten.
I put "morale" in quotes because, for me, backpacking is a fun activity. You talk about the morale of people when they aren't having very much fun. Not enough food usually translates into not a lot of fun.
On a shorter trip, though, not enough food merely means that the meal at the greasy spoon on the way back to civilization could very well be the most wonderful meal of your life.