You are clean when you put them on and you shed skin cells all night. Both facts push the answer in opposite directions, and this is where they land.
The short answer
3–4 wears
When to wash sooner
Wash it now if
- you shower in the morning rather than at night
- hot weather
- you have been ill
What to do between washes
Air them during the day rather than leaving them under a pillow, where they stay damp.
How to wash them
Warm, normal cycle, with your sheets if the fabrics match.