Security guru Bruce Schneier has great advice about when to change your passwords:
The primary reason to give an authentication credential -- not just a password, but any authentication credential -- an expiration date is to limit the amount of time a lost, stolen, or forged credential can be used by someone else. If a membership card expires after a year, then if someone steals that card he can at most get a year's worth of benefit out of it. After that, it's useless.
... An attacker who gets the password to your bank account by guessing or stealing it isn't going to eavesdrop. He's going to transfer money out of your account -- and then you're going to notice. In this case, it doesn't make a lot of sense to change your password regularly -- but it's vital to change it immediately after the fraud occurs.
... 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.