Give him an inch and he will take a yard

The first time I went to London I stayed with a family with two boys of 11 and 13. My landlady was allways saying this to her kids as they were allways asking for more and more of everything (they were a little spoilt boys!)

We've got a quite similar idiom (or proverb) but I like the words from the anglo saxon measures "inch" and "yard" used in the sentence instead of "hand" an "arm".