Research states, that PWS with adult onset (which is called psychogenic stuttering), usually remove stuttering in a year. You can also check [this link](https://stamurai.com/blog/neurogenic-stuttering-vs-psychogenic-stuttering) and [this PDF document](https://www.michiganspeechhearing.org/docs/Copy_of_Psychogenic_Stuttering_-_Diagnosis_and_Treatment.handoutpptx.pdf).