It depends on the site, my school ones for example we have to change every year. I have never changed my password for Eden's. Others it just depends on how much we frequent the site and what the site is used for.
After two identity theft episodes, we now change ours monthly, at every site we use!
WOW! How the hell do you keep track?! Do you use a password program that encrypts them for you and maybe even generates them, too?
I change some of them every three months, others every six and still others once a year and others never. The ones linked to financial things like bank, bill payments, etc. are the ones I change more frequently. Log ons under a fake name for stuff like to check reviews for plagiarism, I don't change.