Donkey work is work which is hard, repetitive and boring.

Examples:

I am tired of doing the donkey work while my workmates get all the fun and creative tasks.

If you want a promotion, you have to do a lot of donkey work first.

Donkey's years is an expression which means 'a long time'. It's used informally.

Example:

I've been living in this house for donkey's years.