Annie Lowrey:

If the United States averages 3 percent annual inflation, and the richest American’s fortunes keep up with Gates’, America would have a trillionaire in 98 years. But now let’s assume that the richest American’s fortune not only matches the rate of inflation, but outpaces it by, say, an additional 3 percent a year. At that rate, we should have a trillionaire in 50 years.

But, as Slate points out, it probably won’t be the United States that has the first trillionaire. We haven’t even had the richest person for some time — that’s now Mexico’s Carlos Slim Helu. 

Also interesting:

The richest-ever American, John D. Rockefeller, had a personal fortune of about $320 billion in today’s dollars, earned during the robber-baron days. And even the Rothschilds, the European bankers whose fortune ran into the hundreds of billions and whose name is a synonym for wealth, likely did not make it to a trillion.