The scan that indicated the 80% reduction was after just a couple of weeks, if I recall correctly.
Since you're talking about immunotherapy not typically being a first-line treatment, I'll share a morbidly interesting fact that underscores some of the, er... quirks of the US medical system. The oncologist treating my relative initially staged my relative's cancer in the electronic health records as stage IV, despite no evidence of metastasis (the usual criterion)--he explained that he did this specifically in order to pursue first-line Keytruda (which is indicated for stage IV but not stage IIIC) and have it be covered by insurance.