Depends for me. Last time between observation and copper treatment they were in QT for a total of 4 weeks. Right now, I'm doing TTM for an engineer goby since it's scaleless and can't handle copper while running a melanarus wrasse and another goby through copper. After TTM the engineer goby will go into a 60 cube for 2-3 weeks for observation. Same with the other two after copper. All three have gone through two rounds of prazi. I'll probably watch them for 2-3 weeks before putting them in the DT. All three displayed some flashing early on but that went away after the first round of prazi.
Regarding copper, I use the 14 days and then into another sterile tank approach and is why I'll likely watch them for at least two weeks. Copper is poison and not enough research has been done, that I've read anyway, that addresses whether extended copper exposure has a negative impact long term on the fish we QT. So I'm trying to limit copper exposure to as short a period as possible and I guess we'll see if it works to eradicate ich/velvet. Just have to test frequently, almost daily, to ensure copper stays at at least minimum levels.