You will never be one hundred percent prepared for an interview. Even fluent people are never completely ready for an interview as you have no idea what they will ask you. The thing that helped me the most was to practice my answers (without practising words) over and over again, each time, my answer differing slightly from the previous time.
Visualisation – imagine yourself in the interview, confident, speaking flawlessly. Visualise the whole interview from the time you enter the building until you leave the building after having had the interview.
Accept that you will definitely have fear no matter what happens and that this is normal. Even fluent people experience fear before an interview, so if you are waiting for the day you will have no fear before an interview, this is not going to happen!
There is no time frame which will automatically render you ready for the interview. When you truly believe that you are able to avoid any stutter with a crutch, there is no situation that will be able to make you stutter. It can be an interview, a speech in front of the world, or simply speaking to a 4-year-old, you will be able to avoid any stutter. This is when you are ready. And guess what? That can be right now if you allow it to be. You are your harshest critic, and if you wait for the day you have perfect speech, you will wake up in 30 years and realize you are still in the same job you wanted to leave 30 years earlier, but were too afraid to.
What is worse than potentially stuttering in an interview? Waking up one day and realizing that your whole life is wasted, all because you were too afraid to take a risk.