Give an inch and he'll take a mile

Give someone an inch and he'll take a mile.

 and Give someone an inch and he'll take a yard.
Prov. Be generous to someone and the person will demand even more. (Describes someone who will take advantage of you if you are even a little kind to him or her.) If you let Mark borrow your tools for this weekend, he'll wind up keeping them for years. Give him an inch and he'll take a mile.
See also: and, give, inch, mile, take