Assume uniform probability of data input transitioning during setup/hold window. Assume setup/hold window is W (nano seconds). Assume clock period is Tc (nano seconds). Probability of any random data transition violating setup/hold is W/Tc (fraction of clock period used by setup/hold window). Frequency of failures is the frequency of the data input transitioning, Fdata * W/Tc. Mean time between failures is 1 over this expression: 1/(Fdata*W/Tc). Or 1/(Fdata*Fclk*W). Fclk is clock frequency. This answer omits the metastability transition decay time.