Well, first of all my friend, I commend your wisdom on QT for your new arrivals; far too many in the hobby have ignored this basic practice and paid the price for it. (yep, even yours truly).
The safe period is usually around 4 weeks. But remember, it's not just the calendar you're watching here; it's an important chance for you to observe the new arrival, and closely. If any problems show up, the time could obviously be extended. Likewise, if you feel as though the subject is doing great, feeding well, etc., you could shave a few days off. If you have a large population of fish mixed with inverts (FOWLR or reef) you should use extra caution, as you cannot afford to medicate your system with copper, and once a fish is in the LR/reef display, they can be REALLY hard to catch and remove without creating a mess of your display.
