With my filter, an ancient partially broken undersized sand filter, it can easily be a week for the water to clear up after the algae is completely dead. For some people with Intex cartridge filters, it can be two or three or four weeks for the water to clear up after the algae is completely dead. We run into this kind of situation fairly regularly, undersized sand filters and Intex cartridge filters are both relatively common.
For everyday green algae, it frequently only takes one, two, or three days of shocking to wipe it out. There isn't any point in maintaining shock level for the extra week or two while the filter is working, as long as you know that the algae is dead. Maintaining shock level the entire time would take three to eight times as much chlorine, or more for some Intex owners, which becomes a significant expense and is much more work.
An oversized DE filter, or even an oversized cartridge filter, will clear up the water far more quickly after the algae is dead. In that situation clear water can be a much more useful indication. You don't have the huge lag between algae dead and clear water that a small sand or Intex filter imposes.
With mustard algae, you can have totally clear water, 0 FC loss, 0 CC, and no visible sign of algae at all, yet the mustard algae can come back within a few days of the FC level getting back to normal levels. The shocking criteria, even the more conservative versions, are no help at all against true mustard algae.
With ammonia, the water is clear the entire time, so you are depending on CC levels, and in the final stages overnight FC loss, as the indicators.
With fairly tough algae that has formed a robust biofilm, the water can be mostly clear, once in a while completely clear, long before you burn through the biofilm. This is most common with low phosphate levels, when the algae doesn't take over the pool but rather grows slowly on the walls. In this situation you often see the algae on the walls, but occasionally it hides somewhere (light niche, bottom of stairs, etc) and overnight FC loss is the only indication that tells you what is happening.