Of course the longer the better. 90 days w/b even better. You do have to weight many things thought....fish generally do go through some stress in a qt since we rarely have the equipment for a long term stay and even w/ frequent water changes you can get poor water quality in between. There’s also some stress created if you have more than one fish in the qt as they are generally on the small.side. All things considered you will never be 100% sure that observing alone, no matter how long will ensure everything is ok. So my experience, and that of many, is all things considered, have had success with about 3 weeks. Now the species matters too, so watch closely for signs of stress and try to go at least 3 weeks or as suggested longer for a species like a PB for reasons previously articulated. Nothing is 100% full proof in this hobby. My 2 cents for what it’s worth. Good luck.