When and if they are not too stupid, they'll never be broke. They have enough money, real estate and other assets to live very comfortably off the increase in value of all that stuff. Real estate is a limited resource, so it's value can only go up over a longer period of time. Same goes for e.g. rare and valuable metals.
Had you bought a bar of pure gold for $1,000.- 50 years ago, how much would that be worth today? Had you bought a cheap piece of farmland for the same price at the same time, how much would that be worth today? Had you invested your thousand bucks in a good, solid collection of enterprise shares on the stock market, how much would your portfolio be worth today?
With the billions of assets they have, I'd say it's almost impossible that they'd ever go broke, unless it's out of pure stupidity.
Oh, and they don't need to sell anything anymore either, the steady increase of value from what they have would be enough to keep them going.
Ayup. Unless you were an avaricious psychopath.