How long bra last?

This is a woman question, how long does bra last? In another way how often you buy a new bra to replace the old ones?

I will buy new bra if I saw them become lose, as you know the bra line can be loose after long term of using it.

What do you do if you have purchase the wrong size of bra? I know some shops will allow you to go back to change it.

Some bras I find that their back is not long so you cannot clasp it.