To me, Elizabeth's behavior is reasonable throughout most of the play. She defends her husband after fighting through serious doubts about his feelings for her. After his affair with Abigail, Elizabeth is right to harbor some doubts about this. Yet she overcomes and, I would say, acts nobly in the end.
When? There are several parts of the play where you might point to Elizabeth's behavior and support that she is reasonable or unreasonable.
As a married woman, I think she is being more than reasonable when she fires Abigail and throws her out of her home. She is dealing as best she can with her husband's infidelity and attempting to trust him again all the while attempting to make him desire her as he did Abigail. It is natural that Elizabeth was a little jealous of the servant girl for stealing John's eyes away from his wife.
Elizabeth again is calm when she learns she has been accused as a witch. She tells John that Abigail will have her killed so that John will be free for the taking. She admits her fear, but she is also confident that her husband and father of her children will do all he can to free her.
Elizabeth is rational when she is brought before the court to tell the truth about why she fired Abigail. Naturally, to guard her husband's reputation, she lies in front of the court about John's affair which does damage to her testimony and to John's attempt to prove the girls' motives.
When John decides not to sign the confession, she backs him saying, "He has his goodness and his name". She will not persuade him to sell his soul in order to keep him alive. She is being the good wife, and while this means her husband's death, it also means his memory is honorable.