A couple days ago I just happened across a point I'd missed for years. I'll have to go back and find where it is. Apparently if you use PeVi on a creature to destroy its Might, if you don't utterly destroy the creature it can heal back the Might in a day. (I would rule against this in the case of Ablating and Bind Magical Creatures since you actually extract vis or the equivalent in those cases.) Now, it doesn't specify this is true for all things with Might, but it is stated for something without giving the thing a power. Also, it is stated when talking about what happens if the Might is reduced to 0, so it seems to be a more general statement from that standpoint, too. I should have written down the page number right then. It could take a while to find it... Has anyone else seen this? Does anyone use it?
This has been discussed before (Search, it's out there, more than once, for ghosts, for demons, etc etc.)
Basically, the door is open for the SG to interpret any creature with Might in almost any way they want.
Some creatures (some demons? some ghosts?) rely on the MM to hold them to the mundane Realm - if they lose MM, they are banished back to Hell (or where ever).
Some creatures (some demons? some ghosts?) have stronger ties here, and so can "regenerate" in a day or a full moon, or once every Samhain or whenever. (Or at least until some more complex condition is met, other than hitting them with a big PeVi hammer, which is uncreative re storyline.)
Some creatures have physical bodies (unicorns? dragons?) and even when their MM is gone, you still have a (large, angry) non-magical creature.
Some "physical" creatures could be so magical that once their MM is gone, they expire. Their bodies might even evaporate, being held together by magic - no vis unless you act fast!
So - it's up to the SG, and whatever makes a good story, and a predictable demon is a weak story element.
I think this may refer to the creature in question advancing its Might Score from the apotropaically reduced value using the proscribed methods from RoP: M or RoP: F (or some equivalent for Divine and Infernal beings) rather than actually "healing" the lost Might, but I suppose the latter option might be feasible in some cases...