As much as possible, I try to wash my makeup brushes once a week. I feel uneasy whenever I think of the bacteria that could be growing in my brushes, especially with the foundation brush and the powder brush. My sisters hate me because I get mad when they try to use any of my brushes, because of that, they irritate me by doing it intentionally.
I used the Sephora Makeup Brush Shampoo before, (you can read about my post
here). I used a lot of it to wash my brushes weekly so it ran out pretty fast. I didn't repurchase anymore, I just used whatever's available around the house to clean my brushes.