Key Management is a vital procedure for the appropriate performance of any cipher system. Of key management sub-items is the key change policy that arouses a quest about the limitations of time interval elapsed before necessitating the replacement of the secret base key. This paper discusses the problem of the base key validity time for two distinct cipher systems ;namely the straight forward(synchronous) and the cipher feedback(self-synchronous) stream cipher systems. Its result is depicted as a graph aiding to either choose number of bits for some key parameters in system design forgiven validity interval duration or to evaluate an already implemented system.