Once a week. Sometimes more for pillow cases. Quilt cover 2 weeks usually, and I dont use a top sheet at all. However, I don't have a routine day for doing it!! So it might be less or a little bit more - depends.
When I was in the RAF I was taught how to make a perfect bed. Unfortunately they didn't teach me how to put a duvet in its cover. Half the time I end up inside the bloody thing myself!