How long do you continue surveillance with imaging and sputum cultures in a patient with NTM with no indications for treatment?  

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.