I said "Not Very Often," and that's because I watch a fish for at least two weeks at the LFS. If it is a rare species or in low stock they will mark it as sold for me so I can watch its progress without worrying about it being sold. It pays to make friends with the owners. I never put a fish into my tank unless I am 100% positive it is the right choice, both species-wise and healthiness-wise. Those two weeks are dedicated to watching the fish and reading about it online, as well as making any adjustments to the tank that need to be done (such as closing all holes for a Ropefish or making a new cave)
I acclimate the fish in a bag for half an hour, and if my aggressive fish are all hovering around the bag in attack-mode, I will QT the new guy or girl until I can find a better house amongst other tanks. If the new guy or girl shows any signs of stress in that half an hour of acclimation, I will QT them.
Finally, I will watch the tank after the new addition for at least 45 minutes to an hour and if anything seems awry I will instantly QT the fish for at least a week.
It's a dedicated procedure and I take purchasing and adding my fish very seriously, and thus far it hasn't been a problem skipping a premeditated QT aside from an outbreak of ich (and that was Clown Loaches, which should have been the exception to my rules due to their sensitivity)