A t-shirt sits directly against skin and absorbs sweat and body oils every wear. There is no airing your way out of that.
The short answer
1–2 wears
When to wash sooner
Wash it now if
- worn against bare skin
- any warm day
- visible marks at the collar or underarm
What to do between washes
There is no between. If it touched skin all day, it goes in the wash.
How to wash them
Cold, inside out. Turning them inside out is what preserves prints and reduces surface pilling.