I lay a straight edge (I use a machinist's rule) on the stone. If you can see light between the rule and the stone, it's time to lap. I lap on a DMT Coarse continuous diamond stone. It does NOT, as "common sense" might indicate, leave the stone surface equivalent to DMT Coarse. If you use a DMT Extra Fine, it loads up too quickly to do any lapping.

I prefer lapping on DMT continuous diamond stones and not the DMT stones with the holes on the surface. These stones are very flat although not as flat as a Shapton reference stone which is overkill in my opinion.

Howard
P.S. Use a continuous flow of water when lapping and do it in the sink.