My QT is 40 gallons with a Biowheel and PC lighting. I'm too lazy to take it down and put it up, so it stays up. I also use it as a frag tank. It has some live rock, Xenia, some softies,snails and 2 urchins.
I QT everything, including inverts for at least 4 weeks. That might sound crazy, but I bought carniverous snails once due mislabled livestock. I only lost the animals that were in QT with them.
If I have an ill animal I set up a 10 gallon hospital tank for treatment. I've only had to do this once in 5 years, so it isn't much of a hassle.