You save $10 by buying the dual pack!
So as an avid Clarisonic user, I always wondered about replacing the brush heads every 3 months as suggested. Personally, I don't want to spend ~$100 on brush heads each year and have found that with proper cleaning and disinfecting, it is not necessary to replace that often. I have gone a good 6 months before replacing mine and they were still in good condition.
I clean my brush head after every use, usually with a make-up removing cleanser to prevent build up on gunk. I also take the brush head off the Clarisonic device to let is dry and aerate better. Each week, I use a makeup brush cleaner the give it a thorough cleaning. This removes oils, bacteria, and product. I can tell it's time to replace the heads when the bristles start to fray out (which takes at least 6 months before noticing it) or have a tinge of color that I can't remove through cleaning.