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10 minutes!! It took me an hours to get both sides of my Norton lapped satisfactorily using wet/dry sandpaper, purely because the 2 opposing corners on both grits were lower than the rest.

I have...

I found the whole thing felt coarse when I got my Norton. Once lapped the 8K side feels like glass, the 4K side not so much.

Another one here shaving off the 8K side of a Norton, no issues other than my own lack of shaving talent & penchant for bleeding.