>At what point has the virus mutated sufficiently so that I don't have an effective immune response?

I don't think anybody really knows, this is an active area of research. What is known is that, the more similar a virus is to something you already had, the more likely your immune system is to be able to respond quickly enough that you don't show any symptoms and don't even realize you were exposed. And if you do show symptoms, it'll be more likely that they will be much milder than for somebody who wasn't exposed previously to something similar.