This is a multi-part (more than the 2) questions so forgive me if I make a longer explanation (my opinion)
1. The common wisdom - and I believe (
@Jay Hemdal can for sure correct me) is that you clean out a QT system between uses - and you do not say add 1 fish in week 1 and another in week 3, etc. Keep them separate.
2. The length thing is a debate that has gone on for years on every aquarium forum. On R2R - the recommendations are posted as 'stickies' on the top of the fish-disease forum. I am not going to go against that - but - IMO I think it depends - where did the fish come from, is it going to be shipped directly from xxx to your house overnight, etc. My LFS QT's fish - medicates if necessary - and I Buy it - I have not had a problem. If you're asking how much to observe a fish in a QT setting (like in a small tank) after getting an online shipment. I would say its impossible to answer. Every time a fish is moved - they can get more stress, different conditions, different tank mates. So for example a fish who is in an LFS in a single tank - looking great and eating - may get sick a week after entering your tank. So - if you're going to go through the effort of making a QT tank - for your fish (great) - I would follow the protocol recommended in the stickies above. IN addition - I would ask whatever LFS (assuming you're not ordering online) - to hold the fish for 2-3 weeks (they can do the observation).
The whole problem (not yours - the question) is that again moving a fish in itself can cause stress (thats you've observed for 3 weeks in an LFS) the other fish in the tank - other diseases that might be in the tank, etc the dealer might be using lower salinity or copper in their tanks - which when transferred to your tank - can cause a problem. Temperature is also an issue. Maybe
@Jay Hemdal can comment as well - he is an expert in QT
