These are amazing and so reasonably priced! I purchased these off of amazon for $9.98 for a pack of 4 (2 boxes). They each last 3 months. I sometimes change mine before then, but usually I don’t and they last and still work great for the whole 3 months. However, for sanitary purposes, they recommend changing every 3 months. So for $10 my Olay Brush will work for another year. Love the fact that they’re reasonably priced and the brush is fantastic!