How often do you change your password?

I am sure most of you having Internet access from home or office or even at Internet cafe. Tell me how often do you change your password. I mean for all accounts like hotmail , gmail or even yahoo email accounts.

I know my friend's hotmail account was hacked so long ago and until today she is not able to get back the account. That is why we need to be careful and often change the password for safety purpose. My friend says that she received email from friend saying that she is trouble and need money to come back Malaysia. But she knows that she is in Malaysia, so a few days later her friend email her using another account to say that her account has been hacked.