I have my seven fish tanks in a dedicated room, so it is easy to have a 20g QT always running. It has sand substrate, and is moderately planted. A couple bits of wood, and a couple rocks. Dual sponge filter. Lots of floating plant cover which is very important to fish. As I say, it is permanently running, but without fish until I acquire some.
I decided on this method simply because I want to introduce my new fish to an "established" tank because for most fish this will settle them faster by significantly reducing stress. Most of the experienced aquarists know this, but for those asking the questions, an established tank is a very different thing from a cycled tank. Considering what the fish have been through getting to my tank, I have no desire to prolong the risk that they will succumb to stress and actually develop something. Introducing new fish to a bare tank is piling on extreme stress; fake plants, a couple bits of decor (wood, rock, whatever) goes a long way to easing the fish; an established running tank takes it to the next level. So far, I have never had to treat fish in QT except the "bread and butter" species when acquired from certain sources (i.e., chain fish stores) that I no longer use.
QT can be four or more weeks, depending upon the species.
Byron.