i have 2 matching pillow cases per sheet set. they get washed with the sheets which might be 1-2 weeks depending on seasons and other things. but, if for any reason i defer changing the bed, i will still change the pillow cases (i have spares).

if you are aware of using the same side of the pillow mostly can you flip the pillow about half-way between bedding changes. if you use an anatomic pillow - rotate the pillow inside the case so the previously "underside" of the case is facing up.

or do you mean the inner cases? i do have a inner cases (with a zip) inside the sheet-matching ones. those i wash maybe about 1ce every third time i wash the sheets. again depends on seasons and personal conditions.

finally, i change/wash all the bed linens if i have been sick, as soon as i feel discretely better.

~ waves ~