The life cycle of Ich can be up around 28 days, but can take up to 40 days, going through its various stages of life: theronts, tomonts, tomites.
Just becasue you don't see the cysts on the fish doesn't mean the Ich is gone - it could just be in a different stage of its life cycle. That's why you hear some people suggest up to 6 weeks of quarantine. And the 6 weeks are supposed to be disease free. If a disease shows and is treated, you might want to start another 6 weeks once the disease is treated.
Two weeks, four weeks, or six weeks, it's ultimately up to you. Just know the risks.