A long time finding Eeyore's age? (7,5)
I believe the answer is:
donkeys years
'a long time' is the definition.
(I've seen this before)
'eeyore's age?' is the wordplay.
'eeyore's' becomes 'donkeys' (eeyore is the donkey from winnie the pooh).
'age?' becomes 'years' ('years' can be a synonym of 'age').
'donkeys'+'years'='DONKEYS YEARS'
'finding' is the link.
(Other definitions for donkeys years that I've seen before include "An age" , "What's evident in old ass" , "A beastly long time" , "Ages" , "forever" .)