How high is "high gravity" when reusing yeast

I've poked around on the forums a bit but haven't found hard numbers.
Per the Wyeast FAQ, they say not to reuse yeast from a "high gravity" beer.

I washed and made a starter from some yeast from a batch with OG 1.090. This yeast had little/no viability. I washed and made a starter with yeast from an OG 1.072 batch and it took a couple days but I got some small amount of activity.

Has anyone found the balance point, gravity-wise, where below 1.0XX you get decently viable yeast, and above 1.0YY the yeast is basically useless?

It depends on a host of things, like ferment temps, type of yeast, IBUs, length of time sitting, etc etc. Generally, you want to harvest from lower-IBU beers that have an OG of 1.060 or less. Even so, you can harvest yeast from most anything if you give it enough time and TLC.

I have no evidence to back it up, but I use 1.060 as the cutoff as well.

I just attempted to use yeast washed from a 9% ABV beer to ferment a 10% ABV beer. It took a TON of aeration and constantly feeding new sugar to get it to ferment. It took almost 24 hours for the starter to go. I finally got it up to high krausen, and now it's happily fermenting away.