password length equation

An equation that determines an appropriate password length, M, which provides an acceptable probability, P, that a password will be guessed in its lifetime. Note: The password length is given by M = (log S)/(log N) where S is the size of the password space and N is the number of characters available. The password space is given by S = LR/P, where L is the maximum lifetime of a password and R is the number of guesses per unit of time.