Considering that, as the years go by, Catherine will not promise her father never to marry Morris, when is her decision never to marry Morris taken?

Catherine is an extremely indecisive person, whose views seem to be swayed by those of the person she is talking to. In chapter 20, she and Morris have a frank conversation about her state of mind—she explains that she very much does want to marry him, but that her father will never allow it, and if she does marry Morris, her father will disinherit her. Catherine says that it isn't the thought of losing the money that upsets her most, but the idea that her father will think she is less "good." By the end of this conversation, however, Morris has convinced her that she should not care about her father's good opinion if she has Morris's—at least, if she loves Morris as Morris loves her.
Catherine's father knows her, however, and knows that his daughter can be swayed by the person she is with. He takes her away to Europe, and Catherine and Morris continue to write to each other. At this point, Catherine is still intent on marrying Morris, but she has made him wait for a very long time.
In chapter 28, Morris eventually decides that he must give Catherine up for the sake of his career. Catherine takes this declaration poorly and says she will come away with Morris to New Orleans. The subsequent argument between the two prompts Catherine finally, at the beginning of chapter 30, to tell herself that "she had given Morris up." Later in the chapter, however, she has changed her mind, and tells Mrs. Penniman that the engagement is still on.
Finally, in chapter 31, Catherine tells her father that she has broken off her engagement; although it is clear that she still feels very torn and would not have made this decision on her own, the wedding is off. At the very end of the story, Catherine seems to have resigned herself to her decision, telling Morris that he treated her badly and they will not have a future together.