Hey there Joe,
I have left my fish at levels as low as .07 for six weeks and never had any noticeable problems. Perhaps it could have some long term, unseen effects internally for the fish, but I did not lose a single fish during, or anytime soon after, the hypo.
But I CAN tell you that one of the real problems is that a protein skimmer won’t work very well at those low salinity levels. So it causes toxins and waste to build up in your tank.