As I sit here, in the chair that I've sat in to do most of my pre-defense chapter writing and revisions, I'm wonder how much more I'll have to revise after my defense.
At this stage, not much. If your committee gave you feedback throughout the process, final edits should be minimal. If you've never heard from your committee, it's hard to tell.
I know there's no rule of thumb about amount or extent of revisions I'll face after my defense, but I'm wondering when I can put this thing down for a while and take a breath. It's been a protracted 6 years and I'm ready to move on (or at least off to the side a bit).
Not long. You have deadlines to meet (more than likely), so I'd take those into account. You may want to take a short break, but I would not sit on the diss long. If I recall correctly, I completed the revisions of mine within a week after the defense.
At this point I'm getting sick of reading the same words again, and again. Is it wrong to approach revisions as pragmatic copy editing if nobody's asked you to do anything substantively different?
No. By the way, that is a very likely scenario. If you worked closely with your committee, there should not be substantial changes at this stage. You'll get those when you submit (parts of) your dissertation for publication;)
Cheer up, you're almost done. Good luck!