For me, the length of quarantine generally depends on how replaceable the fish in the tank are. For my altum angel tank, these fish are very difficult to replace. Thus, I had a 2 month quarantine on them and all stock kept with them. I will have the altums by themselves for another 2 months probably for growout purposes. I have mixed the other stock together to see how they react. With that said, if I were doing a tank of only 1 species of fish or of neon tetras or something I can buy everywhere, probably 2-4 weeks.
This last time, I employed a modified aquarium coop method, and it seems to be working well. First, I got the fish in for a few days fed up well. Then once everything seemed stable, I ran their med trio (general cure, ich x, and eurythromiasin???spelling). I fed lightly to not foul the water and left them for a week. Next, I did a big water change and resumed heavy feeding. After 2 weeks, I ran a levamisole treatment. My biggest concern with the stock I had was tapeworms. A month later, I ran a second round of the trio. At this point, everything seemed very healthy and no issues had displayed since the first treatment. Had an issue displayed, I would have direct targeted that and reset my clock. That last sentence is the most important. If issues arise with any fish, start the quarantine back at day one.
A lot of people don't like to preventative medicate and their are pluses and minuses to each method, but for me, I am going to run a somewhat heavily stocked tank with a 220 gallon system. It would be very difficult to remove a specific fish and costly to medicate the whole system. Again, if this were fish available everywhere and lets say a 40 gallon system, the cost and ease to medicate/replace sick fish is a completely different scenario.
Realistically, everyone has to decide for themselves what is sufficient for their stock and their scenario. From what I have heard, many professional aquariums quarantine for 6 months to 1 year+. Up until 8 months ago, I didn't quarantine. My husbandry was good at worst and I never had an issue....... Then I got an order of fish in and didn't inspect them well enough before throwing them in the tank while the lights were out and I was late to work.... Needless to say, I got an incurable disease in my tank that I'm 75% sure was fish TB, although no one was able to identify it. It effectively crashed the whole system and the fish that survived, I ended up having to euthanize as it wasn't worth risking new stock or giving them away and potentially spreading the disease. From now on, everything sees a quarantine.
Ok, thanks for all that!!! My main tank is only 22 gal and my qt is a 6 gal. My stocking is light in the 22 gal (pair of gbr, 3 rummy nose, and 6 amanos) i bought 7 corydora habrosus and have been in the qt 1 week