Numerical results are presented to characterize the dependence of particle dispersion on the Stokes number, and to identify the appropriate particle response time and flow time scales used in defining the Stokes number. Two particle-laden shear flows considered for illustrating these scales are a planar shear layer and an axisymmetric jet. Two-dimensional large-scale features of these flows are computed by using the flux-corrected transport (FCT), time step-splitting algorithm. Particles of different sizes are injected into the shear layer and their dispersion behavior is quantified by using a global dispersion function. Results indicate that the particle dispersion maximizes at a certain value of the Stokes number, defined as a ratio of the particle aerodynamic response time to the characteristic flow time. It is argued, however, that a correct flow time can be based on the dominant frequencies associated with the large-scale organized structures, and not just the global velocity and length scales. Results from the present simulation and several experimental studies on particle dispersion are used to support the argument. In addition, the validity of the Stokes drag law in defining the particle response time in realistic two-phase flow is examined.