Never heard “donkey’s years” before. Better would be "I haven’t seen you in ages.
“Donkey’s years” is common enough where I live (Australia).
English Translation
I haven’t seen you in donkey’s years!
Why not stick to the direct translation (centuries) ?
English Translation
I haven’t seen you in donkey’s years!
Plain daft to use an idiom in the English translation. No one in France itself has heard of “donkey’s years”.
English Translation
Centuries since I’ve seen you.
I believe that the English translation was first on Tatoeba and then the poor French translator was stuck trying to translate it into something that makes sense for French.
Having these oddball idioms in English is probably quite helpful for learning English from French. I just change the translation (which I believe only shows for me). I do a little added work to word it as I would say it. I especially do that for the Tatoeba translations. I think they are mostly ok, but I prefer to tailor them to my own preferences (as long as I can hold to the original meaning).