What is the maximum duration a husband may be absent from his wife without a valid reason preventing return?

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Imam Ahmad set the duration for absence at six months. This determination is based on the precedent of 'Umar ibn al-Khattab. When 'Umar learned of a woman whose husband was absent fighting for Allah, he sent a woman to stay with her and summoned the husband back. Upon asking his daughter Hafsa the maximum duration a woman can endure separation, she replied 'five months, six months.' Consequently, 'Umar legislated a period of six months for men on military expeditions: one month traveling out, four months residing, and one month returning. Imam Ahmad stated that six months is the reported duration for a man's absence from his family. If a man remains absent longer than this without a necessary reason, some of our companions hold that the ruler should correspond with him, and if he refuses to return, the ruler dissolves the marriage.