When do you change treatment for an asymptomatic metastatic melanoma with a mixed response after at least 3 months of treatment with a PD-L1 inhibitor?  

I would continue this patient on immunotherapy for another 3 months, or until the patient is symptomatic due to the progressing tumor. Delayed responses to I/O therapy do occur in about 10% of patients, so this gives the patient the benefit of doubt.