Yes, parasites can and do go dormant. However, they usually do so when conditions for their survival aren't favorable. The ich parasite is compelled to carry out all stages of its life cycle by being warm and wet. These are the condtions that it "likes" because that is when it is likely to encounter a fish host. While in the cyst, it has no way of determining salt levels so it won't know conditions are bad until it emerges. As long as the tank temp is kept elevated and there is water in the tank, the parasite will not lie dormant for any period of time. So, actually, if you maintain whatever treatment dosage you use in conjunction with a higher temperature for long enough (usually about a week after the last spot goes away) you will clear the tank of ich because there will be no dormant cysts.