In transient structural analysis, the time step has to be small enough to describe the shape of the load impulse. At the same time, the time step should also be small to catch the significant mode shapes in the analysis. Usually, people set the Initial Time Step equal to the time period of the significant mode shape (usually the 1st modal shape) divided by 20. This may require you to run Modal analysis first to find the natural frequencies. The Initial time step is determined based upon the two factors above and pick the smaller one of them. After the Initial time step is determined, you can determine the Minimum time step and Maximum time step by factoring the Initial time step, or set them to the same value as the Initial time step.
See more instructions on ANSYS Help at “help/ans_str/Hlp_G_STR5_12.html#a5iQxq1e2mcm” or Mechanical APDL -> Structural Analysis Guide -> Transient Dynamic Analysis options -> Guidelines for Integration time step.