When does Montag say, "How can I go at this new assignment, how can I go on burning things? I can't go in this place" in Fahrenheit 451?
Montag says this approximately halfway through the book, at the very end of Part II: “The Sieve and the Sand.” At this point, Montag has come to understand what books are, and appreciate a world with books. He no longer has the heart to be a fireman.
Montag says he can’t do the assignment because he realizes that the firemen have come to his house. He does not want to burn his own house.
When Montag arrives at the house, the other firemen ask him what’s wrong.
"Why," said Montag slowly, "we've stopped in front of my house." (Part II)
This is a turning point for Montag. He has to decide if he is going to burn down his own house. In the end, he does. Faber tries to help Montag with his special hidden radio, but it is discovered by Beatty and things get ugly, until Montag kills Beatty.
Note: All books have different page numbers. I have given you the page numbers from this link. You should be able to use that to find it in your book. The quote is on page 51.