I personally QT for 4-6 weeks. Some diseases take longer to manifest themselves. Ick or fungal infections usually show up fairly soon. Bacterial infections, and internal parasites take a little longer to show up. The fish from LFS go through several weeks of stress, From fish farm, to wholesaler, to store. It takes a while for stress related issues to show up. Think of it this way when humans get ill,anxiety,internal illness, or what ever the case it takes time for it to show up after stress related issues occur. PTSD for example. I read some post on here that people are not using the benefits of QT You may luck out for a long time but someday the luck runs out. I use to have people come in the shop and say I don't understand how this happened, I never used a QT tank before. Their luck ran out!. not preaching just relating some experiences from the past. Loosing a show tank of mature fish is a large price to pay for the sake of a few weeks of QT