I have actually had stock investment losses (though miniscule compared to Trump's), and carried the loss forward for several years. You can keep on carrying the balance forward each tax year, deducting from the same kind of gain up to the amount of the gain. For example, if you have a stock you sell at a $100,000 loss, and then make a stock $10,000 gain the next year, you take a $10,000 loss to offset the gain and carry forward a $90,000 balance to the next year. You can also take $3000 max of that carry forward loss against your regular income (which is insignificant for Trump).
You can do this for many years, until the balance is depleted to zero. For a $900K loss to last 18 years, you would have to gain $50 million per year on the average. To pay zero taxes, you would have to make sure NOT to draw a salary (or draw just $3000), which is easy if you own the business(es).
