The procedure is permanent so after 3 to 6 months, you are pretty much hearing the final voice. it could show additional improvement over the first year. The problem is learning to use the voice correctly and not getting lazy and slipping into the old speech habits. If you have been using a trained voice before surgery and have it locked in, locking in the new voice shouldn't be much of a problem. If you take therapy after surgery, you may need months to years in order to lock the new voice in. I had a trained but not very good voice that I used for 35 years and because it was locked in, it was almost impossible for me to use the old voice. I attempted this a short time before voice surgery to understand why my trained voice was so bad and it took several attempts before I could make sounds in my old male voice.
Surgery is the easy part, using the voice correctly is the hard part.