For me, I'd say that "success" would be to feel comfortable and natural in the new way of speaking. When I can focus on slowing my rate of speech, I have pretty much a "normal person" level of fluency. But speaking that slowly does not feel naturally to me, and 99% of the time, I don't remember to tell myself to control my rate of speech. I'm sure if I practiced more, I'd get it solidified into my mind, and I'd consider that a success at that point, when I just do it without thinking about it.