When band gap materials are irradiated with an ultrashort laser pulse, the free-carrier density increases tremendously. The resulting abrupt transformation from an insulator to a conducting material, where a large portion of the electrons is excited to the conduction band, induces huge changes in the optical parameters. The transient nature of these parameters is often modeled based on the Drude model, where there is much uncertainty concerning the Drude collision frequency. As this frequency has a great impact on the modeling results, this work discusses several approaches to its treatment. Additionally, we compare simulation results to experimental data discussing shortcomings of the Drude model in combination with a collision frequency based on Debye screening for dilute plasmas.