Slabs, as integral structural components, play a vital role in load transfer within buildings but remain highly susceptible to touch-off explosions, where explosives are in direct contact with their surfaces. Such high-intensity impulsive loads, characterized by extreme strain rates (103–106 s−1), induce complex phenomena like shock wave propagation, dynamic amplification, localized plasticity, and material fragmentation. Despite the escalating use of explosives in global conflicts, such as in Ukraine, Syria, and Israel, and accidental industrial detonations in rapidly urbanizing regions, limited studies address how varying blast durations influence structural response. This research bridges a critical gap by investigating the effects of touch-off explosions on slabs supported only along two opposite edges, emphasizing the practical need for enhanced blast-resistant design methodologies. The study employs a validated numerical model, benchmarked against experimental results from Zhao and colleagues in 2019 for a 2 ms blast duration, and extends it to explore blast durations of 1 ms, 3 ms, 4 ms, 5 ms, and 6 ms. Using an Eulerian-Lagrangian coupled Finite Element framework in Abaqus, the research captures transient blast-wave interactions, pressure-impulse effects, and localized material degradation. The results show that blast duration strongly affects structural damage. Shorter durations cause localized flexural cracking, while longer durations lead to global deformation and flexural-shear failure. Deformation increases nearly sixfold, and plastic damage energy rises 2.5 times as blast duration extends from 1 ms to 6 ms. Perforation size and crushing also increase significantly with longer durations. These findings provide key insights for designing resilient infrastructure to withstand extreme blast loads.