You are correct in the fact that trophonts for ich only stay attached to the fish for up to 7 days, velvet is even less around 48 hours. The way it works is those trophonts fall off the fish into reproduction. Then they become tomonts and encyst to anything hard in the tank, glass, PVC, algae clip etc. Then those release free swimmers that seek a fish to host and start the process all over again.
So the fish "should" be clean after 7 days of therapuetic copper. However there are still active parasites in the different stages within the tank because copper only kills the final free swimming stage. So it basically creates a shield to prevent the fish from being reinfected.
So you have clean fish, but you certainly to not have a clean tank, if that makes sense.
I would never go less than 10 days of therapuetic copper to be safe, the best practice with additional insurance is 14 days. Even at the 14 day mark you have to carefully transfer the fish because if you get ahold of a tomont, while it's unlikely it will be problematic.