How Often Should I Change My Bra?

Bras should be changed at least once a year because they lose their elasticity and so after wearing for some time, they lose their capacity to give adequate support to the bust.
If you have a large bra size, for example DD to G cups, then it is worth thinking about changing your bras every 6-9 months, since they will lose elasticity even quicker than the smaller sizes and will support you less.
Bust sizes also change and vary slightly with age, so it is important to have your bust measured professionally (many department stores offer this service) to ensure that you are wearing the correct size bra.
Most women do not wear bras which give adequate support and this can lead to problems in later life, particularly if you have a large bust. Hence why you should not just change the bra regularly but be measured for the correct size.

Do you mean buy a new bra, or simply change the one you're wearing? If the former - when the old ones stop feeling comfortable. If the latter - every day.

Having said that, recent research shows that bras actually cause sagging and may be responsible for some breast cancers. Therefore, it's best not to wear a bra at all unless you absolutely have to. Certainly, make sure you go braless for at least 12 hours a day to allow the lymph nodes to drain and the chest to expand normally.

I agree , you need to change it evry year , if your like me ( I'm to lazy to swtich them to pretty colors to match my clothes , lol) you should change it yearly or whenever you think its not holding your chest as much as it is suppose to.

Change them as often as you change your panties.

If you mean buy a new one,  just buy it when yours is too small or isn't working for you anymore, lol your probably not like me .. I buy new ones every 2 weeks, I grow out of them fast