i keep a bra for a year or 2. i have small boobs so my bras don't go through a lot of wear and tear lol. ill throw it out if it gets sweat stains, or really dirty or shrinks. or if my boobs grow (which usually doesn't happen)
It seems that I have hundreds of bras (I don't really), so I don't get rid of them very often. And when they should be replaced is just when I think they've become comfortable so... In reality I replace them when the elastic stretches out, or when the under wire pokes through, or when they get too loose or too tight. When I get a new boyfriend I tend to maybe get a new bra or two, for obvious reasons.
I usually keep it til the under wire starts poking me or it stops doing it's job, or I outgrow it cause it seems like my boobs are constantly growing, and I usually buy 2 new bras every couple of months because I go through a lot of bras.