A mule will work for you ten years for the privilege of kicking you once.

William Faulkner (1897-1962) American novelist
The Wild Palms [If I Forget Thee, Jerusalem], (Ch. 6) “Old Man” (1939)
    (Source)

Variant: "... [V]indictive and patient (it is a known fact that he will labor ten years willingly and patiently for you, for the privilege of kicking you once) ...." (Sartoris (1929))