Age, eon (3)
I believe the answer is:
era
I believe this is a double definition.
'age' is the first definition.
(both can mean a long time)
'eon' is the second definition.
(I've seen this before)
(Other definitions for era that I've seen before include "Age marked by a distinctive character" , "Stage" , "Long period of history" , "quite some time" , "Specific time" .)