When they stop biting.
What species are we talking about? Bass you have to break into two categories, SM & LM. Typically, when the leaves drop, LM fishing has been toast for some time. Whenever you notice the shallows are devoid of all forms of life, then the LM bite is off as far as FF is concerned. SM on the other hand can be caught pretty late into the season on fly gear. I was still fishing for SM last year in november. Bassin is far from over, on the contrary, fall is when the lunkers are usually landed.