That little oval indicator in the center of the brush activates when you first attach your brush head, and it turns red when it's time to replace the head.
Promising review: "After the first day of using it, my face was feeling so much smoother, softer, and more radiant. Then after a few days, my acne started clearing up. I have hormonal acne and it has always been a struggle for me to get rid of it, and every time I stress out, huge breakouts will form right around the left and right sides of my chin. Honestly, I think any cleanser will work with this, especially a hydrating one. The best thing is it lets you know when it's time to get a new brush head. Love it. Save your money, because this thing needs to be in your skincare routine!" —deanambroselove
Get it from Sephora for $99.