Every 2 weeks here.
And NO I don't wear underwear in bed and if I do wear PJs I change them every night.
Sleeping in the same bedding everynight is far different from wearing things close to your skin and intimate areas. For a start, you sweat less at night cause you are less active, thats why you are meant to put on fresh deodarant at night (those special medication ones tell you it is a must). You go the toilet less and you probably end up kicking the cover off several times a night too.
I used to change my bedding every week when I lived with mum and dad but when I moved out my flat is too small to dry a Double duvet cover, 4 pillows, a flat sheet and a double quilted patchwork sheet every week, with having no tumble it is impossible, there would be no room for my clothes and underwear to dry.
I also have a sheet at the bottom of the bed and one at the top for Daytime cover so my cats can feel comfortable on my bed without it getting covered in cat fur, these are changed ever 2 weeks too.