An investigation of barrier breakdown in MgO-based magnetic tunnel junctions (MTJs) submitted to pulsed electrical stress is presented. By studying the effect of delay between successive pulses, we observed that a very pronounced optimum in endurance of MTJs is obtained for an intermediate value of the delay between pulses corresponding to the characteristic time for a trapped electron in the barrier to escape from its trap. A charge trapping-detrapping model was proposed which consistently explains our experimental data. The delay between successive pulses affects the density of electrons trapped in the barrier. The average value in time and the time-modulation of the density of trapped charge give rise to distinct breakdown mechanisms. Our model allows evaluating the MTJ probability of breakdown for different applied pulse conditions. An expected endurance of the MTJs is then derived depending on the characteristics of the electrical stress in terms of delay, amplitude, unipolarity versus bipolarity.