Dynamical trajectory calculations were carried out for the reactions of¹¹B+²³⁷Np and¹²C,¹⁶O and¹⁹F+²³²Th, having mass asymmetries on either side of the Businaro-Gallone critical mass asymmetry α_BG, in order to examine the mass asymmetry dependence of fusion reactions in these systems. The compound nucleus formation times were calculated as a function of the partial wave of the reaction for all the systems. This study brings out that for systems with α<α_BG, the formation times are significantly larger than for α>α_BG, which is caused by the dynamical effects involved in the large scale shape changes taking place in the fusion process as well as due to the interplay between the thermal and the collective motion during the collision process. The calculated time scales are comparable to the experimental values derived from the pre-fission neutron multiplicity measurements.