At my age I ought to know when to change the sheets on my bed. My mother used to change both weekly. My step mother used to wash the top sheet weekly and move the bottom one to the top. I just do it at random. Minium time would be one week, maxium, I am not game to confess.....well, can't remember anyway. Seems not to matter too much when you sleep alone mostly and the sheets are black (not satin) anyway.