You must leave them in actual hypo for no less than 4-6 weeks. Ich won't immediately hop off the fish but will die and the effects will be noticeably decreased after a week or two. Should see great improvement within 2-3 weeks. The day after you see no more white dots on the fish you then do another 4 weeks of hypo just to be on the safe side. The fish will be able to tolerate the lowered salinity(1.009ppm) with no problems, so don't think they will die just from the low salinity.