Des siècles que je vous ai vu.

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) ?

1 Like
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”.