Very important question derives from women saying that they have a bra for a few years "is it still good?". This statemeant can only be true if the bra has been worn few times per year. If the bra is used on daily basis, then there is no way it can last that long. There is no manufacturer that will recommend a bra for longer than 6 months and that's speaking of the top quality bras. For bras that are made for quantity not quality, average lifespan is no longer than a month with frequent washing and no one wants to wear a dirty bra. When I say dirty it does not have to be discolored to be dirty, all it takes is to wear a bra for a day and our perspiration will do the job of making it qualify as a dirty bra. So washing your bra is just good hygiene.
When thinking of replacing your bra, you have to take into consideration the fact that the fabric of any bra can be stretched out. After wearing a bra for some time and putting it through a wash it may be surprising to some that the band size can stretch to a one or two sizes up and that's not good because they no longer feel the same. A good quality bra will take longer to stretch because of the better, stronger fabric and double stitching witch holds the bra the same with stitched thread.
The construction of the bra plays a big factor in the durability of a bra. Another huge factor is the ever changing breast size. Most women don't realize that their breast size changes at least six times through out the year. It may be because of the change in weight, amount of hydration in our bodies or just simply environmeantal shifts.
This tells us that one size will not be forever, but those variations change and go back so if you have 3 bras that you wore for the last 6 months they may be not good anymore and its a good time to think of purchasing a new one. If your bra is damaged in any way, that's a no-brainer it has to be replaced. Even though it may still look OK and seem to do its job, the daily use may cause aches and pain from not having a proper support. Each bra is designed to keep your breasts in place. Unfortunately many designers pay more attention on the fashion aspect of the bras and that is starting to be dangerous for the women around the world.
Make sure that you buy a bra that gives you the support you need and than how it looks. To make a long story short you have to consider buying a new bra within the 6 months of buying your old one and if your bra does not have double stitching (stitches are double and run along the side of each other) than the lifetime of such bra is significantly shorter (1 month) with daily use and washes.