It depends on how often you wash your bras but the actual rule of thumb is 3 bras washed about once a week.  Depending on the quality of the bra you may need to buy them that often but really, I think that if you have them on a rotate out basis so that you get ONE new bra every 6 months that means that each bra is 1 1/2 years old before buying a new one, unless you are buying pretty low quality bras that should work for you just fine.  I buy most of my bras at Target and TJ Maxx and they've worked well for me on that system.  High quality bras should last you longer.

New bras every 6 MONTHS?  Sounds like someone was wanting to make a sale!  For me..finding a good fitting bra is a hard enough task in itself..seems every single one I try on fits completely different from the last..so I try to stick to brands that works best for me when I find a good fitting one.  I do believe every woman should have a good inventory of bras...and replace them as needed!