A long time ( Informal ) (4)
(Other definitions for donkeys years that I've seen before include "A beastly long time" , "An age" , "Ages" , "Yonks" , "What's evident in old ass" .)
(Other definitions for ages that I've seen before include "(What seems) a very long time" , "Donkey's years" , "Awfully long time" , "Gets older" , "Long periods" .)
(Other definitions for yonks that I've seen before include "Ages (informal)" , "Slang for a long time, ages" , "Donkey's years!" , "An age" , "Long time (informal)" .)