The term 'EXTENDED PERIOD OF TIME' can be represented by two crossword solutions: EON (3 letters) and LONGRUN (7 letters). An EON is an immeasurably long period of time, often used to describe the longest divisions of geological time. A longrun is a period of time that is longer than usual or expected. Both terms refer to a lengthy amount of time, but an EON is typically used to describe a much longer period of time than a longrun.