It is narrated from Sa'id bin al-Musayyab (may Allah have mercy on him) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: Any woman whose husband goes missing and she does not know where he is should wait four years, then wait four months and ten days (that is, observe the 'iddah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15566