how often do you clean your makeup brushes?

the other day, i was with my sister, and she was doing her makeup. and i lookd at her blush brush, and it was pretty much stained pink from blush, andi asked her if she ever cleans it, and she said never, because its "such a hassle" she claims that when she does its wet for 5 days. ( which got me thinking about the mildew that must have built up form that much moisture for so long)
i have no idea what shes talking about, so i ask the ovbious question, does she dry it, of let it sit on a towel over night, and she claims that she did, but i dont believe her, i wash my bruhses when i ever remember to, which is about 4 or 3 times a month, the usual once a week bit, and there always dry the next morning.
so i tried to convince her that its better if she does, because, the bacteria that piles up in the brushes causes acne, or bad skin, and she says, "Where'd you read that? cosmo?" in a totally sarcastic voice, and i tried to convince her that its a actual fact, but she brushed it off, and went on with her makeup.
maybe i should have went with "makeup goes on better when you have a clean brush" probably would have considered it if i said that,

how often do you clean your makeup brushes? how do you do it?

PS, i will be writing a blog about the difference in between using a makeup brush cleanser, and the wipes, and using shampoo, and soap, sometime in the future.