Only three Fed policymakers expect the unemployment rate, now at 8.1 percent, to fall more than half a percentage point by the end of next year.
　　If the Fed sees 7 percent as substantial progress, the forecasts suggest continued bond buying through late 2014; if a bit lower, bond-buying could be needed through mid-2015, Ashworth said. Underscoring that interpretation, the Fed said Thursday it expects to keep rates low until at least then.
　　Before the crisis, unemployment was closer to 5 percent.
　　Chicago Fed President Charles Evans has spent the last year arguing strongly for the Fed to tie monetary policy more closely to economic milestones by vowing to keep rates low until unemployment fell below 7 percent.
　　Doing so, he says, would avoid the temptation of backing off from easing at the first signs of economic strengthening. In a series of speeches in recent months, he has likened a strong Fed commitment to low rates to a modern-day Ulysses tied to the mast of his ship, prevented from responding to the siren call of premature monetary tightening.
　　While the Fed did not embrace Evans' 7-percent unemployment rate target, it did adopt his view that the Fed should hold fast to easy policy even after the recovery picks up speed -- a historic shift in policy.
　　DON'T STOP 'TIL YOU GET ENOUGH
　　Any eventual ceiling on the size of QE3 looks sky-high. The Fed may only buy Treasuries and agency-backed debt, and some Fed officials say it has bought nearly as much of the U.S. national debt as it can without impairing the market's function.
　　But with mortgage-backed securities, there is plenty room to run: economists estimate the size of that market at more than $7 trillion, although the Fed would need to avoid disrupting the market's function.
　　Michael Gapen, an economist at Barclays, sees QE3 topping out at $700 billion - slightly more than the second round of quantitative easing, but less than half the first.
　　The impact, he said, could be limited to boosting economic growth by a few tenths of a percentage point, although that could be enough to generate momentum.
　　"If you can keep the economy persistently above trend, then that has a self-reinforcing effect," he said.
　　Combined with the European Central Bank's vow to buy as many bonds as needed from euro zone states -- on condition they undertake reforms -- QE3 "has the potential to be very positive for the U.S. economy," Gapen said.
　　However, he said one real threat to the economy remains: a raft of tax increases and spending cuts that will automatically take effect at the end of the year unless Congress acts.
　　But if lawmakers successfully avoid the so-called fiscal cliff, the outcome for the economy could beat expectations and ultimately trim the size of QE3.