i would ask a reputable marine biologist who has actually documented the dormant phase and decline of marine protozoa... most hobbiest are running off what they saw or heard. 6 months is the longest and is probably not common but it still leaves that level of uncertanty as to what will keep a dormant cyst alive and for how long.
in my current tank i have only used dry base rock that was torched, i have only coral right now but i added a fish in the past. the fish came from an LFS livestock tank and so i can safely assume that ich was present there. now an ich cyst can live as long as there is a fish in the system for it to latch onto (presumably). if it latches onto a fish but is unable to go nuts because the fish is healthy then it will remain dormant and not blossom into the disease we all love so much, but it is still producing. bacteria can't help but split and grow and spread. so since i added the fish i can safely assume that ICH was introduced to my system. now while it is true that i have not had a fish for a long time, my system is probably clean... but when i get the next fish will he be clean? not likely.
we should all run our tanks like it is present. this means careful adherance to high water quality. providing sufficient swimming room and hiding spots for our fish. and feeding correct amounts of the right foods at the right times. Keep the stress of the fish down and you will not have to worry about ich. it really is that simple. kinda like the herpes strain that causes cold sores... we all have it but it takes the right circumstances for it to get loose and go nuts.
