Steps? About a factor of 2-3 in grit size per step. So, say, 100 grit, then 220, 600, 2000, etc.
A larger factor means slower material removal. If you go right from 150 to 600 grit, you'll spend a lot longer grinding on an almost-completely-smooth surface to get rid of the few remaining scratches from the 150. And if you don't remove those scratches before going to the next step, they're not going away, they're going to be there forever. Because the removal rate at 2000 grit is even slower still.
You could do finer steps, like 100, 150, 220, 330..., but now you're spending not all that much time grinding, and a lot more time inbetween, cleaning the surfaces, and you need to stock all these grits, and use them in the right order...
Tim