I'm horrible then, I do a week...
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I do a week unless I see issues pop up within that week, at which point I extend it.
During that week I feed a metronidazole garlic flake and, depending on the fish and its condition, add some salt to the quarantine tank. I may also use Melafix or Pimafix if I think it's warranted. I've also dropped in some ParaGuard in the past.
There are a few varieties of fish I don't quarantine because of their general fragility/ issues with moving them, or because they need a mature environment that my bare quarantine tank can't provide.