To much velvet going around from the wholesalers and then most LFS run low level copper which hides velvet and ich. I run everything through CP unless it is not a CP tolerant fish. Copper for the fish that can't tolerate CP. I have 2 QT setup in different parts of the basement, so over 10 feet apart and separated by walls, no need to worry about aerosol transmission. 14 days of CP or copper, moved to sterile QT and observe for a few days, then 2 doses of Prazi 5-7 days apart. I then monitor for a few weeks and usually toss in a black molly during the observation period. Everything goes through 6 weeks minimum, 8 weeks for tangs minimum. Two 76 day fallow periods within a year made me QT everything properly and take no chances. Especially when the first one was a velvet/ich combo that ended up wiping out all my fish.
I will change my procedure as needed, if I freshwater dip the fish and see flukes, I will run the prazi first. If you are going to observe only and not medicate prophylactically then I would observe for over 30 days. If the fish came from a system with low level copper it could take that long to show any signs.