btw, nice post James.
I don't think this a sufficient definition of a "thru-hike", but I have found (similar to Paul above) that there is an important "boundary" reached after a certain amount of time where you start feeling like you are *part* of the environment instead of visiting it. Colin Fletcher wrote about this quite a bit. I think this is actually more progressive than is sometimes credited, but there is often a specific point where you suddenly *notice* it, and that seems major. For me it is the point where the dirt falls OFF of you as fast as you acquire it and you reach equilibrium. Up to then you feel like you are dirty because you are always getting dirtier. I find it is impossible to feel the (essentially abstract idea) that you are "dirty" once you have achieved that equilibrium, and I suddenly feel like I'm part of the environment, in balance with it. It is hard to describe, but there is a definite demarcation point for me. There is also a definite *mental" version of this equilibrium as well that takes a bit longer.
Then there is the other side of the coin, as soon as you get back to civilization, even if you can avoid the upturned noses of the washed "city-folk" as you can now rightly refer to *them*, once you get to a shower and then turn to look at the pile of clothes you just took off, which are ready to walk away by themselves, and find yourself suddenly utterly repulsed by them - then you are suddenly jerked back in to the "civilized" world view.