If we assume a 50% charge at crisis start, and a rational response by the person using it: * Turn off wifi, cellular data, bluetooth, other unneccesary stuff * Turn screen brightness to minimum * Send only text messages * Don't use phone unless sending or receiving important message

any decent smartphone will last 4 days or more.

Example: I have a midrange Samsung A5 for an office phone (mandatory company rules). With wifi and 4G enabled, but only being used to check the occasional message. It lasts 8 days on a charge.