When does rev dimmsdale's haelth begin to fail?

Reverend Dimmesdale's health begins to fail after he becomes increasingly tormented by his guilt over his secret sin of adultery with Hester Prynne. His physical decline is exacerbated by his internal struggle and the burden of his hidden identity, leading to symptoms of illness such as pallor and weakness. This deterioration becomes more pronounced as the story progresses, especially following the public confession of his sin at the end of the novel.