Brake disc skimming is required every alternate brake pad change and other alternate you need to change the brake disc itself.
Life of brake pad and disc depends on braking and driving habits. Some finish brake pad in 10k km and others run for 40k. Longer you use brake pads it becomes old and start spoiling the brake disc as well.
I think in your case best is, get the disc thickness check that it should be more than 6mm at least if not its time to change the brake disc. If disc are fine then get the brake pad changed as older brake pad become hard and brittle making this issue / noise and plus compromising on 100% braking performance.