Longest Possible Safe FW Dip?

[rastaangel]

To date I have lost 2 achilles, 2 purples, a yellow, a powder blue and a hippo tang in QT, they were the only deaths in the QT all other fish survived. I have always gone to extreme lenghts to stop any parasites in my DT and QT every fish for 3 weeks min but as you can see from my list thats alot of dead fish and lotsa gone money...
So my question is how many of you just FW dip a fish then drop it into your display? How long do you dip? I have always done a 5 min dip before they go into the QT, but I have read anything less then 20-25 min or until the fish is in danger isnt doing a good enough job...
IF I decide to pick up another tang that would be my plan of attack