In The Odyssey, everyone in Ithaca - except Penelope - assumes that since most surviving Greeks returned home shortly after the end of the Trojan War, Odysseus is dead. This legitimises a group of men, the suitors, to court Penelope and claim his kingdom.
War, as is the case with Odysseus, was not the only opportunity for someone to go missing for an extended period of time in ancient Greece. I wonder how long the Greeks would wait before declaring them dead in absentia. How long - for example - a real-life parallel to Penelope would have to wait before remarrying?