The point of QT'ing fish is to hopefully prevent ich from ever entering yout display in the first place.
However, if you aren't running some type of medicated or hypo QT, then you would have to QT for up to 12 weeks in order to ensure that there is no chance of it passing into the display.
11 weeks is the longest ich was recorded to live without a host, so 12 weeks would be needed to ensure eradication.
Plus this would also require the QT of all corals, rock, inverts... basically anything that ich would be able to encrust on, and hitch a ride into your tank on.
I have a PBT, that I QT'd for 4 weeks. Went through black ich in QT, then broke out with regular ich once I added it to the display. I didn't treat him, he cleared it on his own, and is doing great now. After my experience, and doing some research on the topic. In my opinion it is virtually impossible to keep your tank 100% ich free.
So why doesn't everyone's fish have ich? Fish can build up a partial immunity to ich over time, and through proper nutrition their immune systems will become strong enough to fend off the parasite.
This is exactly why a hobbyist will not have ich in his tank for years, then all of a sudden it pops up due to some stress factor. The hobbyist just assumes the stress factor caused the outbreak. When in reality the parasite was in the system the entire time, the stress factor just lowered the fish's immune system enough to allow the ich to gain enough of an edge to infect the fish.
I think a QT period is great. It's a good time to settle in by itself after a stressful shipping, observe the fish for any type of disease or parasite, but more importantly (and in my opinion) it's a period of time that allows you to get your fish fat and healthy before adding it to your display.
Honestly... I think ich is overly feared.
Here are some pictures of my PBT from back in June after he broke out with ich:
Here's a picture of him from yesterday:
