Overheard a conversation between a makeup artist and customer at a department store (a very well-known makeup brand) (of course I will not name drop). The makeup artist explained that the brushes should be cleaned with special cleanser or else they did not last long! Felt like interfering and tell the customer it was a downright lie before she bougt expensive brush cleaner.
Some of my brushes is 12 years old and most 7-8 years old. Have never used anything other than shampoo, hand soap and detergent for my brushes and they are equally good in the hair as when I bought them, so save yourself money and use what you have at hand.
Wash your brushes a few times a week if you use them daily.