Background: Pneumonitis is a widely recognized side effect of immunotherapy with increased incidence in non-small cell lung cancers as compared to other solid organ malignancies. A review of the literature demonstrates an incidence of 5% in patients receiving PD-1/PD-L1 inhibitors. While biopsy provides definitive diagnosis of immunotherapy-induced pneumonitis, clinical symptoms and radiologic features are most utilized with treatment guided by the grade of toxicity. The ability to predict the timing, which patients, and to what extent pneumonitis will present has been a significant challenge. Our study aims to look at the timing of onset of immunotherapy-induced pneumonitis in patients treated with checkpoint inhibitors for primary lung cancer to elicit patterns which may assist in diagnosis and appropriate treatment. Methods: This is a single-institution, retrospective study reviewing patients with primary lung cancer who received checkpoint inhibitors between January 2018 and January 2023. The focus was on the date of initiation of therapy and the date of onset of pneumonitis. Immunotherapy-induced pneumonitis was defined as development of abnormal imaging findings with or without respiratory symptoms following initiation of immunotherapy and graded by the Common Terminology Criteria for Adverse Events (CTCAE). Additional patient demographics were collected to assess secondary patterns. Results: A total of 117 patients met the specified search criteria. Of these, 10 patients (8.5%) met the criteria for immunotherapy-induced pneumonitis. The average time between receiving the first dose of immunotherapy and the development of immunotherapy-induced pneumonitis was 66.5 days (range of 1 to 163 days). The average number of cycles received before diagnosis was 3.7, with most cases occurring after less than 5 cycles provided (7 out of 10). The grade of pneumonitis ranged from 1 to 5. All patients had non-small cell lung cancer, with the most prevalent histology being squamous cell carcinoma, ranging from stage III to stage IV. Durvalumab was the most prevalent immunotherapy (6 of the 10). Conclusions: The timing of immunotherapy-induced pneumonitis, while variable, occurred after receiving less than 5 cycles in majority of our cases, with most occurring after just 1 cycle. Interestingly, durvalumab was the most common immunotherapy in these cases. While our study is limited by the number of patients, the incidence of pneumonitis in our study was close to that of the national average and may, also, be reflective of the timing of immunotherapy-induced pneumonitis. Additional studies with larger numbers of patients are needed.