A Monte Carlo study of transient electronic thermalization in double heterostructures following femtosecond laser excitation is presented. Quantum corrections arising from the short time scales of interest have been explicitly incorporated into the simulations. These include the effects of collisional broadening to account for the finite lifetime of the interactions, and of finite collision durations that retard the thermalization process and impart a nonlocal time dependence to the scattering rates. Details of the phonon modes supported by the binary/ternary quantum-well structures have also been carefully included. Results at low-carrier densities demonstrate that adequate inclusion of the finite collision durations is necessary for an accurate transient analysis.