The attainable upper limit of the laser intensity is a key concern in strong-field quantum electrodynamics (QED). For non-ideal vacuum conditions, the extreme laser fields interacting with the residual electrons could trigger QED cascade—the processes of gamma-photon emission and electron-positron pair production. It leads to strong depletion of the laser pulse hence limits the attainable laser intensity. Since the QED cascade is affected by the polarization, beam waist and duration of the laser pulse, we investigate the effects of these parameters based on particle-in-cell (PIC) simulations incorporating the QED modules. We also develop self-consistent dynamics equations to describe the laser depletion process, which agree well with the PIC simulations. According to the analysis, the upper limit of attainable intensity is about 10²⁶-10²⁷ W/cm^-2 in the considered parameter range. Specifically, the circularly polarized pulses drive stronger QED cascade than in the linearly polarized case under the same circumstances, resulting lower upper limit threshold of intensity. In addition, tightly focused lasers correspond to smaller cascade durations and interaction volumes. Thus, the absorption of laser energy is inhibited, i.e., higher peak intensity can be achieved. Regarding the effect of pulse duration, the depletion energy will be dispersed along larger absorption volume so that the attainable intensity will be enhanced. It should be noted that for extremely short pulses (single cycle), the seeded particles of QED cascade (i.e., electrons and positrons) cannot be efficiently trapped in the laser field, and the analytical model tends to overrate the absorption of laser energy. Regarding the extreme low purity case (i.e., the low electron residual density), the stochastic position of the residual electrons will strongly affect the upper limit of the intensity. Overall, these results offer a guideline for further experiment setups of exploring strong field QED processes and construction of the state-of-art hundred-petawatt laser facilities.