We have explored the effect of transition timescales in planetary nebulae (PNe) formation. We have performed hydrodynamical simulations of the evolution of the stellar winds predicted by theoretical stellar evolutionary models adopting two choices for the evolution during the transition time. The evolution of the star from the AGB to the PN stage has been followed for different main-sequence stellar masses (1, 1.5, 2, 2.5, 3.5 and 5 Msolar). We find that the sizes and velocities of the PNe shells are very sensitive to the adopted timescale during the transition time.