Password length parameters are related by the expression P = L*R /S , where P is the probability that a password can be guessed in its lifetime, L is the maximum lifetime a password can be used to log in to a system, R is the number of guesses per unit of time, and S is the number of unique algorithm-generated passwords (the password space).
The degree of password security is determined by the probability that a password can be guessed in its lifetime.