I've done 16 inches. I have 3 full skeins and still probably a third of the one I'm working on. I actually feel much better now about having enough to finish.
Wait. Forget I said that.
By the way, for an easy way to measure things like this, I took a ball of linen yarn (so it won't be stretchy) and traced around the outside of my poncho. At the end I cut it off and then I measured the string with my metal tape measure (again so I don't have any stretch).
Now I have the yarn to keep with my edging to measure against. I haven't fully blocked the poncho but at least this will get me close and I'll finish the edging once it's been mostly attached so that it lays right.