I completely forgot to answer the main question! My reply is that I don’t know how often I wash my bras — I’d say fairly often. I think I put on a new bra every other day which means the one I was wearing goes into the laundry basket.
I do not use special soap, nor do I wash them in a special mesh bag. I wash them with everything else even if the load includes jeans. Everything in my life needs to be durable — if the bra can’t hang, then it doesn’t belong in my drawer.
The only exception to how often I launder a bra (increase the days of wear) is when I go on trips. I’ve gotten kind of lazy about how much I want to carry in my luggage which is hilarious if you think about how much a bra weighs. If I go on a week long trip, I bring 2 bras. In fact I may only bring two bras no matter the length of the trip because if the trip is longer, I usually stay at an AirBnb with a washer and dryer.
The one weird thing I have about undergarments, however, is that I will bring three more pairs of underwear than days I am staying. Two of my girls do the same thing even though I have never knowingly shared this habit. I’ve never asked the older daughter how she packs. Knowing her, she probably only brings the absolute bare minimum since she’s mastered the art of only bringing what she can fit into a carry-on.