The Federal Reserve’s balance sheet, which is fast approaching $4 trillion in total assets, won’t return to normal until sometime between mid-2019 and mid-2021, according to new projections prepared by central bank researchers.

The research suggests the Fed could go as long as 6.5 years without generating enough income to make annual remittances of cash to the U.S. Treasury as it normally does, though that is in an extreme scenario that the researchers don’t envision.

The Fed’s holdings of Treasury and mortgage securities have soared since it began experimenting with bond buying programs during the 2008 financial crisis. Fed staff economists prepare simulations of how the Fed’s holdings and profits might evolve in the coming years.

In the baseline scenario prepared by the economists, the Fed would not sell its growing portfolio of mortgage and Treasury securities, and would instead let the portfolio gradually shrink as the bonds mature. Fed Chairman Ben Bernanke suggested that was the Fed’s preferred strategy at a press conference in June.