Every time you have worn your bra from 2 to 3 times then it's time to wash the bra.
Bras can be worn up to 3 times before you have to wash the bra for hygiene purposes.
I wash my bra after I wear it 2 times in a row.
It all depends on the persons lifestyle but you should not wear the same bra more than 3 times without washing it before wearing the bra again.