I don't have a designated QT tank yet, I have a 2.5g tank that I use to medicate if needed. I use my 10g tank with my female guppies as a QT tank. I typically keep them in QT for 3 weeks unless I see something out of the ordinary, then they stay for another few weeks. Once I get my 30g high I will use my 10g as a strictly QT/hospital tank.
I didn't QT any fish for a year, never had any problems. Then I decided to add 3 Red-eye Tetras to go with my two (at the time) Diamond Tetras. I added them to the tank the same day and within a week my dwarf gourami, pleco, two guppies and all the red-eyes were dead. My gourami was the coolest and I really miss that fish. I can't find any near the quality my original one was.