characters recover health at 18 health/minute
characters recover health at 24 health/minute
characters recover health at 30 health/minute.
The following characters heal at double the rates quoted above:
Wolverine (Astonishing X-Men, Patch, X-Force), Daken (Dark Avengers, Classic), Deadpool, Spider-Man, Hulk, She-Hulk, Blade.
So, essentially, anyone with a healing factor in comic book mythology heals at double the rate. Oddly enough, Rocket and Groot, even though they have true healing, don’t heal faster, but you can always take them into a match, and yellow match them back up anyway. You can calculate time to full recovery for a specific character using the rates above.