Buy them as often as you feel is appropriate. I usually buy around 2-3 new bras a year, but that's more of a...my size keeps changing issue.
6 to 9 months could be resonable time to change. Our marketing strategy also says same. This was earlier 1 year period. But advent in mfg. technology and research have increased production much beyond leaps and bounds requiring reduction in recycling period. There is a big anamoly. How about increased life cycly out of advent in technology? If you don't consume fast, then every shop will be just full of underwares only or mfg will come to halt one day.
Sounds like you're not wearing the right size.
Every 180 days. That's how long the average bra is supposed to last. But your preferences could differ