My answer to the question is preferably once in a week at least. But, my mom changes the entire bed linen every three days diligently!! I make it a weekly excuse saying I live alone but I do pass on for a month or two sometimes as well.
This ensures hygiene for one. If you have oily and acne prone skin, using same pillow everyday might result in breakouts so changing the pillow covers once in two days might help with controlling the breakouts. And, also, for hair, clean bed covers help to control dandruff and keep oily hair clean.
Also, if you might have noticed, the day you spread freshly washed bed linen, you get a good night’s sleep. You feel fresh when you get into the bed. Also, the room looks dirty and unclean if the linen has not been changed for long.
Oh and if you are down, you have to for sure change them after you are UP So, its a good practice to get the bed clothes washed at least twice a week. That’s on my next to-do list
So, how often do you change the covers?