Frequent password changes have been proven counter-productive to good password security, and they should only be changed when a user deems necessary, such as in the indication of a data breach.
Passwords with mixtures of letters, symbols, and numbers are not necessarily safer, as people often choose predictable combinations. A better solution is to choose a password consisting of a few random words because a phrase is often harder to hack than a small combination of letters and special characters.
A good practice is to screen new passwords against lists of commonly used or compromised passwords to ensure your password is secure. [Test your passwords at haveibeenpwned? today!]