30 days minimum with an additional 14 days to make sure fish is OK.
Running hypo is a running a fine line. If its too low it will kill the fish, too high and the Ich lives on. Evaporation will drive your readings nuts trying to keep it right.
Just run a QT and use copper for 30 days with another two weeks copper free to observe. Check copper levels everyday and adjust as needed.
If you have it in your show tank you may want to try No Sick Fish. $57.00 for a small bottle, but it seems to work.
QT and take it slow is the only way to avoid losing livestock.