So, a little more clarification. You should be posting when you have read about 20%, then 40%, then 60%, then 80%, then 100% of the novel. Basically, talk about what your thoughts are so far.

Some things to consider: What do you like? What do you dislike?
What do you notice about the author's style?
How do you feel about different characters?
What incidents stood out to you the most and why?
Are there parts you find unrealistic, humorous, tragic, etc.?
What things don't you understand?
Can you personally connect to the novel in some way? Does it remind you of other things you've read or seen?

Be honest, try to explore thoroughly, and you can't go wrong. You can (and should) also respond to what others have written about. Happy blogging!