The growth I've seen is as variable as the horse's hooves. They grew much faster when the grass was lush, and have slowed way down as the winter hair started coming in.
I don't know if it's because my Saanen is light colored, or if it's a breed characteristic, but he keeps his feet pretty well in trim with just a little help here and there. They have a lot of rough climbing rocks and concrete, and ground that varies from marsh to exposed limestone. His hoof wall seems so thin after the hooves. I've only had to use a file on it.
BTW, my double-palomino paint (stock horse type) has the 2nd hardest hooves in my heard. They may be white horn, but they are like stone. Only the solid black arabians have harder horn.