No QT is perfect, so stop looking at QT as a silver bullet. What you're after is healthy fish.
The main point of QT for healthy looking fish is simply to feed and observe them.
If they're is good conditions and are well-fed, these fish never get sick and end up getting transferred to the display.
Two weeks would be a fine rule of thumb for a time-frame of doing this. More or perhaps even less as you see fit.
If you can watch a fish for two-weeks in a LFS and are satisfied with that, I wouldn't necessarily feel compelled to QT them even more at home.
However, I don't know how often someone would be able to monitor a fish for two weeks at a LFS without the fish getting purchased out from under them. You could expect it to take some patience.
On the flip side, if you happen to be well-equipped for QT at home, I've known folks to come to the LFS on delivery day to pick up their fish still in the wholesale bag. I wouldn't necessarily say that's better per se, but it's smart in that circumstance because it exposes the fish to one less traumatic round of handling and possible exposure to another set of pathogens.
If all you can afford is a bare-minimum QT setup, I would say don't bother.....healthy fish will be coming out sick due to the stress of being in there.