Give an inch and he'll take a mile
give (one) an inch and (one) will take a mile
If one makes concessions for someone, that will embolden that person to take further advantage of one, instead of being content with what they have been given. If you let the kids stay up later on the weekends, they'll want to do it all the time. Give them an inch and they'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.