We performed a time dependent percolation analysis of the degradation phenomena in ultra-thin CoFeB/MgO/CoFeB magnetic tunneling junctions. The objective was to understand the microscopic degradation physics of coherent tunneling and the thickness limitation of the MgO barrier. We propose two models: a trap assisted tunneling (TAL) model and a filamentary defect assisted leakage (FAL) model. The correlation between resistance drift behavior and barrier lifetime was then calculated and compared with real data based on these models. The relationship between the resistance drift behavior and barrier lifetime was found to be well explained by the TAL model, the random trap formation in the barrier and the percolation path formation which lead to barrier breakdown. Based on the TAL model, the measured TDDB Weibull slope (β) was smaller than the value estimated by the model. By removing the effect of some initial defects in the barrier, an ultra-thin MgO tunneling barrier in MTJ has the potential for a much better lifetime with a better Weibull slope even at 3ML thickness. This method is rather simple but useful to deeply understand the microscopic degradation physics in dielectric films under TDDB stress.