I'm not sure.... I've had bra tops where the underwire poked through and that means it's time to throw it out. But that happened just a month or two after buying it.
I've had some Gap bras for several years now.
I had some bras I bought a long time ago and I threw them out because they're too small now.... I used to weigh a lot less because I had health issues.
I've never "worn" a bra out though, except a cheap one from wal-mart that had goofy all over it when I was a teenager. haha. But all my bras are high quality.
I also have to wear special types because I have an uneven sternum. When I was a baby I had really bad asthma and it caused my sternum to get wack, so I have to have the fabric in the middle pretty wide--so I can't wear Victoria Secret bras.