2 -4 is what I've heard folks use.
I can never seem to get past 2 1/2 weeks when they eat garlic and Zoe soaked food like pigs and look OK for that amount of time.
I'm sure there's gonna be someone to tell of a horror story when only QTing for shorter periods, but I tend to buy hardy fish only. Also, I tend to buy most fish from the LFS
after watching them for 2 - 3 weeks at the LFS
in the coral tanks - which means they are not in copper for long periods and look very healthy.