I just started hyposalinity treatment for ich in a 40 breeder. I haven't read anything that says how much fresh water to add at a time. I know the total time frame is 48 hrs. I was going to do maybe 5 gallons of fresh RO every 8 hrs.
If your fish are in tough shape I wouldn't wait 48 hrs to get to hyposalinity. Fish can handle the drop in salinity much easier than bringing it back up after the treatment. You're going to be surprised how much salt water will need to be replaced with fresh to get to hyposalinity. Make sure you have a refractometer or Pinpoint salinity monitor to hit and stay at the target salinity range. Good luck.