If you are not certain about the fish's health, and we rarely are, you need to use your QT for at least 2 weeks of health. That is long enough that if there is something developing in the fish, you will see it before you get it in your main tank. If some disease develops, you need to treat it and then wait another 2 weeks after it seems healthy to make sure of no new symptoms developing. If you only wait a day or two, you might as well put the fish right into your tank. You will pick out the obviously healthy ones before you get them at the store and acclimating them twice in as many days causes stress that will give you no benefit.