The individual has a big impact here. I went to therapy for several years in my teens. I saw minimal improvement. I went back to the same therapist in my 20s. Over the course of more than six months, we developed my fluency. I was done with therapy in using my fluent speech in the real world in less than a year. I can't tell you when, but at some point after achieving fluency, you just don't think about your stutter anymore. The anticipation recedes down to a memory.