My personal experience about doubling the sanding time when changing grits doesn't hold up. I do a lot of high gloss sanding going from 220 to 4000 grit on 7+ coats of poly finish. I do enough of this on the same thing that I built an adapter so I can mount my random orbit sander to my CNC. I programmed a toolpath to follow a spiral path (because it's a round object). The path starts from the center, goes to the outside and then back to the center where it lifts the sander and I replace the pad for the next grit. Because the CNC is doing the sanding, this means I have the exact same time per grit on the same path with the same speed and pressure. I do not have to double the time when switching from 1000 to 2000, for example; the same time removes the scratches from the previous run. My observation may only hold true because I am sanding with a CNC and not by hand.