It depends on the breed and hoof wall stimulation and health of
the hoof really. But in general, 1/4-1/2 a inch a month. Drafts
tend to grow hoof wall a lot faster. I work on a draft cross mare's
hooves and she grows about an inch of hoof wall in four weeks.
For Howrse Archimedes question: 1/4 - 1/2 inch. :)