Sorry to hear about the other fish but good news that the tang is doing well.
Hyposalinity should be treated for about 2 weeks straight to ensure any reproductive cysts have "hatched" out and there is no longer any danger of free swimming theronts
. The "timeframe" if you will is about 4ish weeks depending on how well the treatment worked, if performed correctly. The first few days are for acclimating and slowly lowering the salinity, two weeks for the treatment and then a few more days to slowly raise the salinity back up. At the end once the salinity is back to normal and the fish appears to no longer be infested, it's best to wait another week for observation and allow the fish some extra TLC time to regain some of it's strength.
It can then be properly acclimated
back to the main tank.