I go a minimum of 4 weeks with an average of 6 weeks. It doesn't cost much to setup a good QT system and it pays big dividends in the long run. You get to know the fish better during this time and establish a baseline for it's behavior. It also lets the fish adjust to the meals you provide without the added stress of competition. I always cringe when you see someone complete their big tank build then purchase all those fish they always wanted at once, throw them in, then you read the post a couple weeks later of ICH and sickness running all over their tank. It also gives you time to establish a stocking list order for introduction into the tank. I always try to put the largest and most aggressive fish in last.