Let me just start by saying the Norton flattening stone works fast. Of course, it's possible I could have just lucked into a very nearly flat 12K, but still...
I drew a pencil grid on both sides, wet down both stones, and did about 30 seconds on each side to see which was closest to flat. They were about the same, so I arbitrarily picked one and went to town.
I stopped periodically to rinse off the 12K and the Norton. when I finally got that side flat (all the pencil marks gone and the entire side in contact with the Norton) I flipped the 12K and went to town on the other side.
Time from drawing the pencil grids to having both sides flat? 30 minutes. I was a bit surprised, considering the 12K's pretty tough reputation.
So, since the razor I was testing on was still in pretty good shape, I only did 4 laps on the DMT 8K. No worries there.
Moving on to the 12K. I wound up doing about 25 laps there. I don't have a magnifier to check the edge, but I was watching the water wave in front of the blade. By about the 22nd pass I was starting to see another very small wave appear in front of the main mass of the water being pushed by the blade, so I assumed at that point the edge was about there and stopped.
To preface - the hanging hair test doesn't work very well for me. My hair is pretty coarse, and even a wicked-sharp razor from a honemeister, or straight off a .25 micron paddle, won't pop one of my hairs. The razor DID slice off the hairs on my arms, though. I guess I'll find out how well I did in the morning.