More of a general topic than PCT specific, but absolutely relevant for this group.

When you go backpacking, how many days on trail does it take for you to need to increase your calories? Is this days or weeks? Do you find it depends on the terrain, that if you're going up and down mountains that it hits you much sooner than if you start in a relatively flatter section of trail?

And when you do, how much more calories do you need? Would you say it's 50% more? 100% more?

Food can be the single heaviest item a backpacker carries. At 1.5-2 pounds per day it's going to add up bigger than any of your big four items even just carrying three days worth. Making this the single biggest item where carrying enough is important, but also not carrying too much.