There are a wide range of pathogens and afflictions which ornamental marine fishes may harbor. Some are intestinal/internal that take weeks to make an outward appearnce to the human eye. Other pathogens like the Marine Ich parasite, require weeks to go through their life cycle, then multiply in large enough populations to have an effect on the fish confined in a small space, then days or weeks more for the hobbyists to see them on the fish.
For instance, if the QT is infected with a Marine Ich cyst, it could take up to 6 weeks for the hobbyists to see the first spot. This is allowing enough time for the parasite to go through its cycle and show up in enough numbers on the fish to be seen by the human unaided eye. The next generation of parasite during the next two weeks would show an 'explosion' of the parasite that couldn't be missed by the hobbyist. In this case, the hobbyist would hold the fish in quarantine for 6+ weeks with the showing of a spot on week 6. So 'spot-free' for 6 weeks gives about a 99% confidence level that this parasite isn't there in the QT.
Although a longer quarantine time does increase the confidence level above 99%, from experience and knowledge of the pathogens most likely to find their way into the QT, they will show themselves within 6 weeks. Lengthening the time in quarantine would not seem to be a good investment, or would not have much of a payoff, if any.
If the hobbyist is not observant and doesn't spend a lot of time with the fish, or leaves town/away during the process (and the care taker doesn't know what to look for or doesn't care), it is certainly reasonable to extend the time in quarantine.