Generally I wash the bedsheets every week, or every other if I get lazy. I also tend to take a shower before bed so I'm definitely clean. Towels I also wash about every week, and I just alternate between 2 during the week so I end up using each one about 3-4 times. Wikipedia says that that's 'normal' for the US. I know if someone else wasn't changing my boyfriend's sheets it would be a lot longer between washings, and don't even get me started on the towels.
How often do you wash sheets and towels? When do you consider them dirty?
[Image from Prickly Pear Flickr]