Denim degrades from washing far more than from wearing. Every cycle pulls indigo off the yarn surface and abrades the cotton.
The short answer
10 wears
When to wash sooner
Wash it now if
- a smell that survives airing
- visible dirt or a spill
- the knees have stopped springing back
What to do between washes
Air them overnight. Spot-clean marks with cold water and a cloth, working inward from the edge so you do not spread a ring.
How to wash them
Cold, inside out, gentle cycle, minimal detergent, line dry.