I'm finding my sanding skills/results lacking, and wonder if I'm doing something fundamentally wrong.

Some questions:
1) Say you've got a 5 or 6" ROS with excellent dust collection and high-quality paper (Mirka or Festool, etc). Assume you're sanding a large table top (or something where 100% of the sandpaper is contacting the work). How much time do you put on a sanding disc before you consider it "worn out"? (Seems like a new disc quickly loses a bit of sharpness, but beyond that, is it usable for 30 seconds or 30 minutes?)

2) How thoroughly do you vacuum/brush off a surface when switching between grits? Is it: "Not at all - the (Festool) dust extractor gets it all", or a "I spend several minutes with the air compressor blowing every last remnant of the last grit off"?

3) How much time do you spend with each grit? Say you were sanding a completely-flattened coffee table top (~2'x4') through 120, 150, 180, 200.... is that a multi-hour procedure or a ~15 minute procedure?

4) Finally, would you do any hand sanding of a flat surface, or is the ROS the last step? Even using what I think are "best practices", a sideways-aimed flashlight shows sanding scratches that I'm worried will show up after finishing, so I end up using a sanding block to do the final sanding "with the grain". Is that normal/necessary?