Well, when we make things in our world, say a flashlight, it can only keep producing light until the energy stored in it runs out, unless you then give it more energy. But if you want a rock on top of a hill, it takes a lot of energy to get it there, but once there it stays there until something else affects it. So, maybe being unplottable is more along the lines of being on the top of a hill.

The sort of spells that seem to run out seem to be the sort that are more active. Containing someone who wants to move, speaking, etc. Whereas the sorts of spells that seem to last are usually, but not always, more just ways of something being. Artifacts seem to usually be embued with energy at time of creation, and I think if they are low energy needs, they will last long enough for it to be effectively forever for practical purposes.

However, I'm more curious about the conservation of identity... those paintings of real people are kind of creepy... and how does that work. Bits of a person are replicated and they can gain information through where their images are... and then aspects of this linger even after death. I think that's far weirder than any minor mass/energy oddities.

When Ron and his family were in the paper, did they have a huge spy network of all the places where the paper was? Ron didn't seem to have any access to the info from his picture's locations... maybe learning how to use that is more advanced magic.