The running-in phase is the first stage of the bearing lifespan. However, this phase is very short and extremely important for the future lifespan of the rolling bearing because it is what sets the stabilized state in terms of roughness of the parts in contact, residual geometry and surface residual stresses, which are key factors in the fatigue resistance of mechanical parts. Several numerical and experimental studies have highlighted the importance of the running-in phase in two scales (macroscopic, meso and microscopic). Due to its high flexibility, the approach presented in this work is a numerical modeling of the running-in phase which has been based on the Weibull distribution. The obtained results confirm the importance of the running-in phase on the lifespan of bearings or other mechanism whose functioning requires an adaptation phase of parts in contact. It also concludes that if the running-in phase has been performed correctly, there is a marked improvement in reliability. The curves describe the useful saved time of lifespan according to the scale of the running-in phase.