I feel like this is a topic that for many is an afterthought; we all know we should probably do it more often but are lazy.
For me, i aim for weekly but usually ends up fortnightly.
Pillow cases are the biggest case for regularly washing for me; you sink your face's oil and dead skin onto them for 8 hours a day.
You wouldn't reuse underpants for 30 days so why would you reuse a pillow case for that long.
I wash them every time I take them off the bed…
but I think you are trying to ask how often do you 'change' your bed linen?
the difference between my underpants and my pillow is that I don't stick my naked crotch on my pillow.
I changemy wife changes them weekly