We used scattering dichroism to study the dynamics of dipolar chains induced in magnetorheological suspensions under rotating magnetic fields. Both the dichroism (proportional to the total number of aggregated particles) and the phase lag show different behavior below and above a cross-over frequency. The cross-over frequency depends linearly on both the square of the magnetization and the inverse of the viscosity. The Mason number (ratio of viscous to magnetic forces) governs the dynamics. Therefore, there is a cross-over Mason number below which the dichroism remains almost constant and above which the rotation of the field prevents the particle aggregation process from taking place. Our experimental results have been compared with particle dynamics simulations showing good agreement.