Final answer:
Er was dead for twelve days before he returned to life in Plato's 'Myth of Er'.
Step-by-step explanation:
In Plato's 'Myth of Er', Er was lying dead before he returned to life for twelve days. This specific detail is outlined within the narrative of the myth, which can be found in Plato's work The Republic.
In the context of the 'Myth of Er', the twelve-day period signifies the transition phase between death and Er's reawakening, serving as a precursor to his recount of the afterlife and the process of reincarnation.