In 7-9 days If you bring down salinity too 1.009, and then they exlpode and die.
Hypo is great if you can maintain temp, low salinity and small water changes to keep amonia low. The good thing I found out by doing hypo is that when fish have Ick, they have trouble breathing. With lower salinity comes higher oxygen and it becomes "easier" for them to breath. This I found a big plus when treating for Ick, as the fish is less lathargic, and more ready to eat. Feed high protein tiny pellets daily and suck up any not taken, leave a few in the bottom. Take out 10%, add 10% 1.009 for three weeks, but the last week increase by no more than .002 until you are back to DT level.
Least stress better ability to breath, I like this method most.
I copper when the specimum, is covered head to tail.......just my take