Donkey’s years
Definition 1 of 1
Pro Tip 1/3Very Informal
Used mainly in casual conversations among friends or family, not in formal situations. 
I haven't seen you in donkey's years! How have you been?
Pro Tip 2/3British Origin
Primarily a British expression, understood in the UK but may confuse people from other countries. 
He's been with the company for donkey's years, since the very beginning.
Pro Tip 3/3Exaggeration
It's an exaggeration, it doesn't mean a specific number of years, it just feels like a really long time. 
I've been doing this job for donkey's years, but it never gets easier.