In this case I will say never.
It is completely impossible to implement. Sure, a few financial assets held by pipsqueek investors have values that can be determined, but beyond that lies chaos.
A large stock holding, say 15% of the float) cannot be liquidated at today's market price. Its value is much less because selling will drive down the market. So what is its value? The only way to find out is to sell. If Buffet sold his interest in BRK the whole company value would tank, far beyond just the price pressure from trying to move his shares.
This is why Berkshire would immediately begin to pay a large dividend - a 25% tax on 15% of a company requires a 4% dividend once you are past the first year the accumulated basis would make the tax less than the dividend
Real estate assets, same impossibility to value, especially for minority partners. A limited partnership in an apartment building, for example, cannot be sold for its nominal fraction of the appraiser's number. It can only be sold at a discount.
And how to value a non-public company? A yacht? A collector car?
These is already stated on if non-market information is not available tax will be paid when sold but at much higher rates
The government simply does not have enough attorneys to fight valuation battles with tens of millions of people and, anyway, there are not enough courtrooms.