How do you know if you have enough sources?

[XemaSab]

I am working on a project about a religious group that involves a lot of original research.

The amount of written material that has been written about my subject is fairly small, like 40 books or so about their beliefs plus probably another 10 or so background books.

Since they're a religious group, though, they have thousands of blog entries and sermons online.

If I stick to the books, I don't think I will have enough source material. If I go to the blogs and sermons, there's just so much.

How do you know when you have enough to go with and how do you know if you need more? Also, how do you know when to just let it go and assume you have enough stuff to go with?