Every two months my bank makes me change my password to view my statements online. I take 5 minutes that day and change my passwords to everything. However, I make sure that the password to my social networking accounts, my e-mail accounts, and my bank account are all different. If everyone did it this way, it'd be a hell of a lot harder for someone to get a hold of another person's account of any type (provided, of course, that the passwords you set aren't obvious).
Only one I tend to not change is the password to get on my computer. I should probably get on that.