Hi,
do beings with might heal wounds at the same rate normal people do?
If I break a giants arm with a spell, is it still broken a month later when I return with reinforcements?
If one of my grogs gets a magical lion in the eye with an arrow, is it still injured the next day?
There is some argument for beings healing wounds as they recover might pool.
Faeries will definitely heal in an instant if that's what their role demands. The wound is just a prop to the story.
In The Mysteries, Appendix A (Immortal Magi) gives guidance on how to handle what happens if a magus gains a might score. I would say these guidelines (any temporary, unbound change will fade completely over a season) would apply to most immortal Magic creatures. (As mentioned above, Faeries heal depending on their story role). Page 135 of TMRE suggests you roll weekly for wound improvement for all non-incapacitating wounds, and if the roll fails the wound is still automatically stable with a +3 to the next roll.
Demons, on the other hand, have to heal normally.... if they choose to keep the physical form that was damaged. If they drop their physical form, however, they can then re-create themselves, perfectly whole. As doing that requires only a few turns and a single point of Might, it's pretty darned easy to do. (RoP:I, pg. 29). Note that if their physical forms are killed before the demon can drop it, the demon is actually killed permanently.