I'd say the speed varies between an iron and a coal rock. Sometimes I can one-hit, other times it takes a few hits. I'm only 66, so you'd get it faster most likely. Most of the time, the world will have someone in it already. But after some world hopping, you can usually find an empty world. The rocks respawn pretty fast. If you have them all to yourself, it'd take maybe 10-30 seconds for the 1st rock to respawn after you 'kill' the last one. I may be off on this respawn time though, been a while since I mined there.