If you keep your tank fishless then it needs to be at least 6 weeks.
Copper is toxic to everything just that the fish can withstand higher concentration than the whitespots, therefore i guess copper treatment is out for me.
Using copper in your quarantine tank will also not allow you to quarantine corals in future
Copper is also very unstable in seawater hence you need to monitor the concentration of the copper solution carefully else the concentration will drop and the whitespots will not get killed.