The frequency for me is two weeks. Don’t be lazy, they would be more dirty than you thought. The cotton sheet and bedding accompines us closely in the night for a long time, during which time, your body would emit a lot of toxic things and tiny unseen germ, it will be much better if you put them in hot sunshine every once in a while,but the case is usual not like this, there are often gray days without sunshine such as in rainy seanon or cold winter, then the bedding, if not cleaned after one monthes or even longer, it may be harmful for your health in a way unknown to you.
Two weeks is great, one month is ok too, according to your habits. I remember when I was living in school apartment, where the living condition is poor and damp, and I don’t quite change the sheet that often, so I got some skin problems at that time, but when everthing changed and I moved to better places and have good living habit, those problems are nonwhere to see.
So if you got skin problems, first check if you wash your sheet and cotton bedding in a required frequency.
