This paper aims to investigate the characteristics of durations of discretionary lane changes (LCs) on freeways based on an enriched dataset containing LC vehicle trajectories of 2905 passenger cars and 433 heavy vehicles. A comprehensive analysis of LC duration is conducted and four stochastic LC duration models are built according to vehicle types and LC directions. It is found that the LC duration varies across different vehicle types and LC directions. The modelling results show that different variables have different effects on LC duration for different vehicle types and LC directions. Fixed-parameter, latent class, and random parameter accelerated hazard time (AFT) models were built considering driver heterogeneity. Results show that heavy vehicle drivers show more heterogeneity. Different variables were found for different vehicle types and LC directions. The results of this study can be beneficial to understand the mechanism of LC process and the influence of LC on traffic flow.