Good article from Bruce Schneier on Changing Passwords.
So in general: you don’t need to regularly change the password to your computer or online financial accounts (including the accounts at retail sites); definitely not for low-security accounts. You should change your corporate login password occasionally, and you need to take a good hard look at your friends, relatives, and paparazzi before deciding how often to change your Facebook password. But if you break up with someone you’ve shared a computer with, change them all.
Two final points. One, this advice is for login passwords. There’s no reason to change any password that is a key to an encrypted file. Just keep the same password as long as you keep the file, unless you suspect it’s been compromised. And two, it’s far more important to choose a good password for the sites that matter — don’t worry about sites you don’t care about that nonetheless demand that you register and choose a password — in the first place than it is to change it. So if you have to worry about something, worry about that. And write your passwords down, or use a program like Password Safe.