hello there. It seems like it's common for people to change their bedsheets and pillow cases once every 2 weeks. It's the same for me. Sometimes, i will change it weekly though. It really depends on my mood. =D
There are three bedrooms, two for guest and one for us. If the sheets are not dirty, dusty, then in guest rooms it stays for a week or 10 days, whereas in our room once in 3 days we put it for washing.