What is Mr. Bennet's reaction to Mr. Collins's assurances that he will visit again?

At what point in the book? Certainly, Mr. Bennet does not expect Mr. Collins to return once Elizabeth has refused his proposal of marriage. And his initial visit is made to "preen" in front of the Bennet family since he has come into some wealth through the benevolence of Lady Catherine. He is hard to take seriously because he is somewhat pompous, but Elizabeth will not marry him and so the reader does not take him all that seriously.