Personally I don't do that. Actually I bought a little 5 gallon tank with that intention but needed to move some tiny fish in there instead. Plus it was getting so that most of the fish I was buying would be far too uncomfortable in a 5 gallon tank and probably would die from stress.
People have used those large plastic Rubbermaid containers for quarantine (at least 2-4 weeks probably) add a filter or some air stones and a small heater.
I've always carefully looked over my fish before adding them into an aquarium and haven't brought home anything contagious yet. I also limit where I buy my fish so I'm not getting fish from all over town (actually most of my healthy fish I bought online) most of my locally purchased fish have at least partially died out) - just my experience. The problem with buying a bunch online is that you can easily overwhelm your aquarium cycling with too many fish at once - but shipping is costly so you tend to buy a lot at the time. Or buy smaller groups and suck up the shipping costs (recommended). They usually have a much bigger variety than anyplace else. LiveAquaria.com and azgardens.com
You are looking for:
White spots or velvety patches on the body
- Missing scales or incomplete fins
- Cloudy or bulging eyes
- Ulcers or other lesions
- Rapid breathing, gasping at the surface and shaking
- Erratic swimming
- Listlessness or sulking in a corner