The selection of an appropriate timestep size is essential in order to obtain good convergence rates for your simulation. A general discussion on when a steady-state or a transient solution should be attempted is available in Steady State and Transient Flows.
Note: For eligible cases, there are Stabilization Control settings to automatically reduce the timestep size or time scale value and reattempt any timesteps/iterations that fail to converge or that exhibit solution divergence. For details, see Stabilization Control in the CFX-Pre User's Guide.