I only recently started to have the ability to remain focused on a book long enough to get through them in a week or less.
For me, it also depends a great deal on the author. On a Kindle I am more likely to give up on than a traditional book, but you can't argue with the portable nature of it.
if I am not at least somewhat excited about what is going on in the story by around 15% of the way through a book, or less if it is some super-long monster of a book, I give up on them and walk away.
I've not heard of these clubs. Have you looked into a local library ebook system? most places have them. They have time limits as to how long the book will work on your system but here's a trick, check the book out, then take the kindle offline and it will remain in there until you put it back online again.
Yeah, we are members of a library, but I don't think I can read the books quick enough. I might give it a try in the future. There is a series I like that I think the books are too expensive to buy, so I think checking them out will be my only option.