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
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • accompany (one) on a/(one's) journey
  • accompany on a journey
  • a stranger to (someone or something)
  • be out of (one's) league
  • be out of somebody's league
  • be in bad with (someone)
  • (one) puts (one's) pants on one leg at a time
  • bargain
  • bargain for (someone or something) with (someone)
  • brief (someone) about (someone or something)