I was thumbing through my books looking for spell guidelines when I stumbled on this bit. Since I remember it being inconclusively discussed even recently, I figured people might still be interested.
There you go, the RAW for Might recovery, at least as far as immortal beings (such as demons) are concerned. 
Now, of course, if somebody else has already reposted that bit, I'll just crawl back to under a rock. 
I seem to recall to also have seen it somewhere in RoP:I or its divine sibling - with a recovery breakdown into hours even. I think it might have been together with mentioning of mythical effects to enhance the speed of recovery.
Now that you say it, I recall as well, although I'm at a loss to say where, and too lazy to check 
Read the intro to the Appedix (pg 133), however, and you will see:
"This appendix provides common rules for immortal magi" and later (3rd paragraph) "A being of one of the realms... blah blah does not of iself make him immortal" thus the rules contained don't apply to demons (as I read it).
Demons are immortal souls.
I have to disagree on that point. ROP:I Pg 28 Spiritual Form of a Demon: they are infused with mortality. Ageless, but not immortal or unchanging. They have souls like humans, but the form that visits earth is a spiritual form that can be killed, and I maintain, stripped of it's Infernal Might Score (permanently).