know when you are well off idiom
知足
If you do not know when you are well off, you do not understand that your present situation is good compared with other people's or with what it might be like.
She's always complaining about her car - she doesn't know when she's well off (= she's lucky to have a car even if it's bad)! 她总是在抱怨自己的车不好——她根本就不知足。