I can't seem to find the answer by Googling, and it's bothering me a bit too much.
Does he regenerate lost body parts or lost stone/skin? Never read comics that much so I have no idea.
I have found no listed source of a healing factor. And, to quote the other question linked above:
In a battle against Wolverine (Fantastic Four vol. 1, #374 (Mar. 1993)), his face was cut open by Wolverine's Adamantium claws and bled. The claws left scarring that Ben would later wear a helmet to hide the scars. He was later healed by Hyperstorm.
On the other hand, he does heal normally, probably at a human rate, as he has had chunks of his stone skin knocked off which have grown back, and he apparently does not age as discovered in Fantastic Four #605, which suggests some level of cellular regeneration (or possibly simply no cellular mutation or shortening of telomeres).