If you are going to hypo in case new fish have ich, then you need to do the all out procedure, which is 3 weeks in hypo, and then about 4-5 days bringing the QT back to normal salinity. If you do less, then you would not have addressed ich adequately, and the ich that the fish may have, may develop a resistance to the hypo conditions or maybe the ich never reached the free-swimming stage and therefore never was adequately treated.