While I was pulled in by the first month, I have retained sponge club for about six because it is a great way to make sure to throw away your old sponges. I keep an extra one in my car for frosty mornings and bring some to the office. I even keep a few extra in the bathroom and am always reminded to rotate them out. Sure, the sponges will individually last a month or two then start to break down if you don't change them at all- but that is the point! You aren't supposed to use any sponge for a month or two. I have seen the light on sponge rotation and I am not going back.
PS: I subscribe to a lot of things like Blue Apron, Birchbox, ect. This has been the most practical.