I think the 1mm/hour refers to accretion depth-wise across the entire existing diamond surface area. So conceivably (but probably subject to other factors that

I think the 1mm/hour refers to accretion depth-wise across the entire existing diamond surface area. So conceivably (but probably subject to other factors that would complicate matters) if you started with a .001 mm thick diamond slice with 4656 mm^2 surface area and applied this process on one side then it would take 3 hours to get a 3mm-thick diamond slice.