Perhaps your worries about hygiene arise from watching too much soft-core porn in which women get in the shower and soap their breasts for extended periods. This is not because breasts are intrinsically dirty but because such scenes are gratifying to dirty minds. If bra hygiene were an actual issue, don't you think American womanhood would have been subjected to decades of commercials along the lines of, “What's that smell?” “It's Myrna's bra. Someone has to tell her!”
On your behalf, I actually polled some of the cleanest women I know on their bra-washing schedules. The answers ranged from “weekly” to “when my white bras look black and can walk themselves to the washing machine.” The average was a monthly laundering. So your girlfriend's behavior is perfectly normal and neither unhygienic nor gross.
Your friendly neighborhood Lingerie Addict washes her bras once a week in the spring and summer, and once every two to three weeks during the fall and winter. I have several favorite “everyday” bras so I rotate them during the course of the week; each bra only gets worn once every 2-3 (or more!) days.
The general rule is that the more you sweat, the more often you should wash them, as sweat not only decomposes the fibers, but could also rust the underwires, and even result in a lingering “sour” smell (one that doesn't go away).
Other tips include not wearing the same bra more than one day in a row (bras need time to air out as well as regain their original shape so as not to lose elasticity), always washing your bras by hand (though if you do use a washing machine, always use a lingerie wash bag), never machine-drying your bras, and using a special lingerie cleanser when you wash your bras to help them last longer.