Since NTM lung disease typically develops over years, it is reasonable to monitor the patient with periodic HRCTs, even if the patient is relatively asymptomatic. I typically do this every 12 months in an otherwise stable patient. The reason for this is because we know that in up to 2/3 of patients,...

The length of follow-up would depend on whether there is underlying bronchiectasis, CF or non-CF, comorbidities including diabetes, connective tissue ILD, immunosuppressive medications, and the specific NTM. In my experience, it is even up to a decade. 

I agree with continued surveillance with CT. I would advocate for somewhat more frequent sputum cultures, as changes can occur and might be significant.