What is an ideal maximum password length?

Bruce Schneier has a couple of interesting articles on password policies and password length. Real world passwords https://www.schneier.com/blog/archives/2006/12/realworld_passw.html - covering password length, complexity and common passwords. Password advice https://www.schneier.com/blog/archives/2009/08/password_advice.html - some do's and don'ts including:

DO use a password manager DO change passwords frequently. I change mine every six months... Don't reuse old passwords.

DON'T use passwords composed of dictionary words, birthdays, family and pet names, addresses, or any other personal information.
DON'T access password-protected accounts over open Wi-Fi networks — or any other network you don't trust — unless the site is secured via https. and many more Changing passwords

 https://www.schneier.com/blog/archives/2010/11/changing_passwo.html - a detailed discussion on how often you should change based upon usage and threat environment.