High-power laser processing shows increasing importance in the manufacturing industry. Solid-state lasers provide optical powers of several kilowatts in continuous-wave mode with power densities of more than 1 MW/mm2, thus helping to achieve economically relevant processing speeds. However, to minimize health risks due to intense laser radiation, sophisticated safety concepts are required. An essential part of these concepts is the laser-process housing, which typically consists of metallic walls as passive shielding against laser radiation. The standard EN 60825-4 defines requirements and testing conditions for these shielding materials. Here, it is considered that the material durability depends not only on the laser irradiance on the material surface but also on the laser-spot size, which is attributed to hindered heat conduction at the spot edge due to heat accumulation occurring at larger spot sizes. However, this behavior has not been fully understood. In this work, a simplified finite-element modeling approach based on the heat equation is used to simulate the dependence of the material durability on the laser-spot size for 2 mm thick structural steel, a typical shielding material in industry. The calculated times to reach the material-melting temperature are compared with the measured material lifetimes upon laser irradiation. It is shown that the presented finite-element modeling can reproduce the general size dependence of the material durability. Thus, this analysis to calculate the times up to the material-melting start can be used to derive lower limits of the material lifetimes under defined irradiation conditions, suitable for designing the shielding sufficiently.