Timestep factor for calculating natural frequency of tunnel-rock model- Dynamic analysis

I have a circular tunnel at the center of a model 100mmx100mm. The surrounding rock medium has joints in the form of bedding planes and Voronoi blocks. Gravity is applied and the model is brought to equilibrium. Before applying dynamic input motion to the base, I would like to know the amount of damping I need to apply i.e., the parameters of Rayleigh damping. For this, I want to calculate natural frequency of the model. I have applied a 1000 Pa stress on the top of the model and I am checking its vertical displacement at various points along the depth. I will need to run analysis for at least 1s to check the number of cycles and thus, calculate the natural frequency. However, UDEC takes a very small timestep 1e-9. It is taking extremely large number of time steps ~10^8. This will take significantly large time. Is there a way to increase the timestep? Can I use block mechanical timestep-factor fx fy for increasing timestep? Say fx = 0.5, fy = 3. Please guide. Thank you.