POLL- How often do you change your bed sheets?

My grandma seems to think that it's normal to change your bed linens once a week and that it's gross if you don't. I say that would be a waste of time. So, how often do you change your bed sheets?
Update: Wow, lots of once a week'ers. I change them whenever I start to kick the fitted sheet off my mattress, which usually takes a few months. Teenage state of mind, perhaps.
45 answers 45