I'd say above 10 times the national average income, 100% tax rate. In France, during this election, this measure was proposed by Mélenchon's Front de Gauche (100% tax on income over 35,000 euros per month). Hollande, the PS candidate, talks about a 75% tax on income over 1 million euro per year.
There's a myth that I've seen written over and over on this site. It's the typical "only the rich invest" logical fallacy. People don't get the nonsense. They should understand "only a society with savings from a great mass can invest on the long term through a well-regulated banking system"