Narrated 'Umar (RA) regarding a wife of a lost husband:

She should wait for four years and then observe the 'Iddah for four months and ten days (before she may remarry). [Malik and ash-Shafi'i reported it].

وَعَنْ عُمَرَ ‏- رضى الله عنه ‏- ‏- { فِي اِمْرَأَةِ اَلْمَفْقُودِ‏- تَرَبَّصُ أَرْبَعَ سِنِينَ, ثُمَّ تَعْتَدُّ أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا } .‏ أَخْرَجَهُ مَالِكٌ, وَالشَّافِعِيُّ [1]‏ .‏