Bullard said he expects U.S. growth will gain enough momentum to let the central bank reduce the pace of bond buying as early as the middle of the year. The unemployment rate will fall to the “low 7s” by December from 7.9 percent in January, which would meet the FOMC’s test of the “substantial improvement” needed to end purchases, he predicted.

“It is going to be very difficult for them” to end QE3 without triggering a rise in borrowing costs, which may argue for sticking with the program until the economy has sustainable momentum, said Carl Lantz, head of interest-rate strategy at Credit Suisse Group AG in New York.

Japan is one guidepost for Fed officials as they consider how long to keep stimulus in place. The Bank of Japan lifted the benchmark lending rate off zero in August 2000, before an economic recovery had consolidated. Monetary-policy experts, including former Fed Governor Frederic Mishkin, have called that move a “mistake.”

“All throughout the crisis, Japan has been on their mind,” said Mark Gertler, a New York University economics professor who has co-authored research with Bernanke.

Numerical Thresholds

Investors eventually will have to take the Fed at its word: The federal funds rate isn’t going to rise until the unemployment and inflation goals are hit, Gertler said. If the economy drifts further away from the thresholds -- as it has in the most recent reports -- then investors should begin to expect a longer period of rates near zero, and that provides continued accommodation.

While the end of QE3 is tied to subjective criteria about the labor market, policy makers have set specific numerical thresholds for raising rates. The FOMC said in December, and repeated in January, that “an exceptionally low range” for its benchmark would be appropriate as long as inflation isn’t forecast to rise more than 2.5 percent and unemployment remains above 6.5 percent. These criteria replaced previous calendar- based guidance that rates would stay near zero at least through the middle of 2015.

“The market is always looking for the next big thing,” said Lou Crandall, chief economist at Wrightson ICAP LLC in Jersey City, New Jersey. “Once the Fed stops buying assets, everybody knows what the next big thing is: the beginning of the exit cycle.”

Advance Signal

Even though Bernanke said Dec. 12 that the transition to economic thresholds “doesn’t change our mid-2015 expectation,” money-market-derivatives traders since have accelerated their time frame for policy tightening.