To investigate the influence of the duration-temperature pairs chosen for laboratory thermal maturations on the evolution of the organic matter (OM) and porosity of the clay-rich source rocks with increasing maturity, long duration (104 days) thermal maturations, bulk, molecular geochemical OM characterization (Rock Eval, GC-TCD, GC/MS) and nitrogen adsorption porosimetry were applied on six type II mudstones from the Kimmeridge Clay formation. These results were compared with shorter duration experiments (72 h) previously carried out. The results show that the increase in the duration of thermal maturation experiments enhanced the degradation of heavy polar OM components into saturated and aromatic hydrocarbons, leading to the production of similar amounts of extractible OM (EOM) enriched in saturated and aromatic hydrocarbons. Then, in preventing the early breaking of low energy bonds, the lower temperature used for the longer duration experiments led to a less pronounced early cracking of OM resulting in a different timing of gas generation. This process did not influence the general evolution of the pore volume, which increased during gas generation. Whatever the duration-temperature pair chosen for maturation experiments highly oil-prone samples formed lower pore volumes during gas generation. Increasing the duration of thermal maturation seemed, nevertheless, to influence the amplitude and the timing of pore alteration events that occur during gas generation, leading to slightly different pore size distributions and pore volumes between short and long duration experiments. However, these differences remain limited, suggesting that results from different time-duration experiments can be easily compared.