I guess you quantify the result in steady state. In such case, the duration time is not important. the important thing is how much power is used in the simulation.
If you check the transient result, it should change with time under the same optical power source. Please use a point monitor to record the temperature vs time and see if it is expected. Please check the mesh settings, time step sizes, and ensuring that the material properties are accurately defined, especially if they are temperature-dependent.
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