Although Damsluver's info is correct, not everything will go by the book and the 17 days is not going to mean the same for all tanks.
I also disagree about the cleaner shrimps. Sometimes they might have no affect and sometimes they do a great job.
In my tank, the fire shrimp gives my tang a daily cleaning and has removed ich from his body on many occasions. So its not correct to say that cleaner shrimps have no affect because they do alot of the times.
Ich is always present in our tanks. The 6 week life cycle doesnt really mean much, ich will come right back if the fish are stressed. If the fish is eating, the chances of recovery is very high and there is no need to put the fish thru more stress by removing it from the DT. In my personal opinion, i think fish that are not eating and show other signs of stress should be medicated. Just spots on the fish is not enough reason to remove him if all other signs are good (eating, acting healthy, etc.).