I try and wash sheets and pillowcases once a week or every other week. I would prefer once a week but I don't always have the ability to do so. Good question, not sure what is considered normal.
I call the cover of my duvet also sheets (in dutch but still) It's best to do it all at once (obviously), otherwise you have to think when you changed this or that.
Of course. Maybe they also call it sheets in English too, I don't know. But if they don't, maybe you shouldn't either, because we can't guess what you mean.
I would have but I didn't thought of it before you mentioned the cover. I just never think about this stuff in english so I just assumed when nothing else was mentioned you all too included the cover when you say sheets
I wash my sheets usually once a week. Sometimes I go 2 weeks depending on how busy (read: lazy) I am.
I change the sheets and pillow cases every week and when it is summer usually twice in a week. My duvet cover and blanket and mattress pad, every two weeks and I have my duvet (down) and throw pillows cleaned twice a year.