pablo... That sucks. Sorry for the losses.
Clint... 6-8 weeks is even shorter than professional zoos and aquariums do it. They go 90 days (so roughly 12 weeks). I don't know any hobbyist that does that. Most hobbyists go for 2 weeks, though the more experienced do tend to lean more towards greater numbers like you have heard. From what I've read, most diseases (not all) reveal themselves within the first 3 weeks, so I aim for 3 weeks myself. Now, if any disease shows up, I reset the clock after the disease seems completely gone. That being said, my last addition only went 2 weeks and 4 days. The only reason why I shortened the time was because I was going out of town soon and I didn't have someone I felt comfortable with monitoring a fish in the QT. So I set him free at the beginning of my weekend so that I had two days of pure observation to make sure he adjusted well. He adjusted too well actually. Moved in like he always lived there.
Also note, that a fish isn't entirely disease free at any time, just like humans. They always have diseases in them, even after being treated for it, but if you see no symptoms their immune system is keeping up with it. This is why a fish gets sick when it is stressed. Their immune system dropped and the disease that was in their system already takes advantage of it.
I know you were probably looking for a more definite answer, but the truth is the correct answer is to set the fish free back into the ocean. We aren't doing that, are we? Heheh... So you just have to make the best judgement that you feel comfortable with. Who is more stressed about the QT, you or the fish? If he can wait out 3 weeks at the least, that is my suggestion. If you have the will power to go longer, more power to you!