The first thread that Big G linked is the thing to read. I have been in the "observation only" camp in the past, keeping fish in QT for 30 days and watching them like a hawk. I can tell you that I have spent a total of hours a day watching for behavioral signs, like scraping, twitching, etc.
Then, when you see stuff like that, you will wonder what to do about it, was it something to worry about, or not? If you are the kind of person that can spend the time and deal with the detail, you can go with observation for 30 days, maybe 45, and no copper in the tank. You can also run two rounds of prazipro for flukes, just to be sure, because it is easy.
What you shouldn't do though is start copper, or run it below the full dose, and then stop before the full testament period. Doing that might just hide a problem, not solve it.
If and when something shows up on your fish, you will need to treat. In my case, 20% of the fish showed an issue inQT that required treatment. Some of those showed signs right away, some after 3 weeks. At that point, you start treatment, and the clock starts over. Lately, I am running in to more issues, and I am thinking I might just start treating with copper from the start. You will also read about chloroquine phosphate, an alternative to copper. If I could get my hands on that, I would prophylactically treat QT with that, every time!
And not everything will be parasitic - some problems could be bacterial, or viral, or behavioral. Those seem to show up more quickly though, and will have definitive solutions such as antibiotics, baths etc.
Read that first link ....