- Quality/initial cost
- How frequently they are washed
- How much time they spend in a closet vs on the bed
Higher quality sheets will last longer, tighter weaves will resist unraveling longer. Washing your sheets breaks down the fibers and machine drying them provides heat and friction which wears them down further.
Generally a good rule of thumb is to replace your sheets, cotton or otherwise, when they become stained, frayed or faded. Some people feel the need to replace their sheets every year or two, while others can have their sheets last five, ten, even fifteen years depending on usage.