<h3>Purpose/Objective(s)</h3> Overall treatment time (OTT) prolongation may be associated with decreased overall survival (OS) in stage III NSCLC. We aim to further delineate the association between treatment prolongation and OS in stage III NSCLC in a large dataset, and hypothesize that there may be a specific range of OTTs where higher radiation doses are beneficial. <h3>Materials/Methods</h3> We queried a large national dataset for patients diagnosed with stage III NSCLC who were treated with chemoradiation (CRT) using standard dose schemes (59.4Gy-66.6Gy) between 2004-2017. Non-prolonged OTT was defined by number of prescribed fractions, plus weekend days and 2 additional days. Patients were stratified by total days of OTT prolongation: non-prolonged, 1-3d, 4-6d, 7-9d, 10-12d, 13-15d, and 16+ d. Patients who received 60Gy were compared to patients who received 66Gy, stratified into 1-3d, 4-9d, and 10+ d. Multivariable Cox proportional regression models were used to investigate the association between OTT prolongation and overall survival (OS), adjusting for patient demographics and clinical characteristics. <h3>Results</h3> We identified 26,101 total patients, of whom 8,644 had prolonged OTT. 46.5% of patients were prolonged by 1-3d, 18.6% 4-6d, 12.9% 7-9d, 6% 10-12d, 4.5% 13-15d, and 11.2% 16+ d. Using no OTT prolongation group as a reference, each prolonged group had a statistically significant increased HR for death. The HR for each group progressively increased as the magnitude of treatment delay increased. Furthermore, each subsequent group trended towards a greater HR of death when compared to the group directly prior, though not all were statistically significant. Among patients with prolonged OTT, 3,809 received 60Gy, 1,924 received 66Gy, and the remainder received other dosing. Those prolonged by 1-3d showed no difference in OS when comparing 60Gy to 66Gy. For patients prolonged by 4-9d, those receiving 60Gy had a significantly increased risk of death compared to 66Gy (HR 1.2 [p = 0.0052; CI: 1.06-1.38]). Patients with 10+ d prolongation no longer saw a significant benefit with a 66Gy dose (HR 1.16 [p = 0.065; CI: .99 – 1.36] 60Gy vs 66Gy). <h3>Conclusion</h3> In stage III NSCLC, any OTT prolongation is associated with increased HR of death. The magnitude of risk increases in positive association with magnitude of OTT prolongation. Increased RT dose (66Gy vs 60Gy) may compensate for modestly prolonged OTTs, thus conferring a survival benefit in some patients. This benefit is not significant in cases of minimal (1-3d), or severe (10+ d) prolongation.