I remember watching an intervew with a philosopher dedicated to answering those questions. She says that, in her country (Norway or somewhere near, I can't remember), surveys show that the majority of citizens consider having 2 million dollars the max amount a family of 4 should have before being "too much money any family should have". So she suggests putting a cap at 10 millions for an individual. In her country, that's officially more than enough while not detering people to try to become "rich". Big emphasis on "in her country". She believes it should be very dependant of the amount and quality of public services. Good quality and cheap public education etc.