Elastic needs a rest day to recover its shape. Wearing the same bra two days running does more damage than one extra wash would.
The short answer
2–3 wears
When to wash sooner
Wash it now if
- it has been worn twice
- a hot day or exercise
- it no longer sits level
What to do between washes
Rotate between three so each gets a day off. That single habit extends the life of all three more than careful washing does.
How to wash them
Hand wash or a mesh bag on delicates, cold, and never tumble dry — heat is what kills elastane.