Yes, but I prefer 10 days because that would also account for the front end of ich's known lifecycle. However, there is no margin for error if you go this route: Therapeutic levels must be maintained for 10 consecutive days, no cross contamination can occur from the treatment tank AND the sterile holding tank the fish gets transferred into must be housed at least 10 feet away. And of course, the fish should still be observed post transfer to ensure symptoms do not return.
Are we talking about in the DT or QT? In the DT, the lifecycle gets interrupted by going fallow and thus denying the free swimming stage a (fish) host to feed upon. So you're essentially starving it to death. In a QT environment, you can use a chemical (such as copper) or hyposalinity (only works on some strains of ich) to "zap" this same free swimming stage. Neither provides any direct benefit to the fish, other than to shield them from reinfection. This is why the 10 day treatment and then transfer method (described above) can work; because you are waiting out the time the trophont can remain on a fish before dropping off, shielding your fish from the possibility of reinfection and then transferring the fish to another tank. Thus leaving behind any lingering parasite problems (i.e. unhatched tomonts) in the original treatment tank.
However, if you only have 1 QT to work with then treating for 30 days becomes the next best thing. The vast majority of strains of ich and all strains of velvet will complete their entire lifecycle in 30 days or less. You are just observing post treatment for symptoms of ich in case an unusual strain has been encountered. 30 days of copper treatment works probably 95% of the time, assuming therapeutic levels were properly maintained for the entire 30 days.
P. S. The reason velvet requires a 6 week fallow period (and not just 30 days) is because in a DT (reef) environment the dinoflagellate is capable of using the aquarium lighting for photosynthesis, thus extending it's life before starving to death.