This is how it looked after 20 turns. The second set of ten I do is in the opposite direction. I find that I rarely need to go beyond 40 turns even on real bad ones and 30 seems to be the magic number.
DO NOT try and get any of the deeper corrosion spots out of the circular area left from the valve being closed. You would need to remove a lot more bronze than is necessary to accomplish this.
You are looking to get the most contact area you can without over grinding and enough to keep the valve from weeping in either the open or closed position.