FLAC calculates critical time step for stability by itself. One has to give a time step smaller than the critical time step so that the solution converges. If you give a time step larger than the critical time step, the solution will diverge or an error message will be displayed.

Thank you for your answer. But what are the factors on which a critical time step depends upon? Can you kindly elaborate.

The critical time step depends on the combination of mass (or inertia) and stiffness of the system.

If any system or structure) is having high stiffness, the critical time step will reduce and you have to choose a time step less than the critical time step for your analysis which will take more time.

Try a parametric study on reducing the interface stiffness between two surfaces (if any in your problem) so that the time step increases without having any effect on the results of the simulation.