I keep them in Hypo for 4 weeks from the last visible sign of Ich. So, if you go for 2 weeks with no ich and then you see some again, reset the clock. Keep in mind that this 4 week period does not include the transition days from regular salinity to hypo and back. Really, the longer the better. It would be a real shame to go to all the work for this and not break the life cycle of this parasite because the hypo or fallow period wasn't long enough by a few days or even a week.
Here is a good read, it also has a link to hyposalinity treatment
http://www.petsforum.com/personal/tr...marineich.html