The Fed will regularly review the policy and could adjust the amount of buying up or down depending on the economy's path, he added.
In a rare televised interview, Bernanke told the CBS program "60 Minutes" the Fed's actions are aimed at supporting what is still a fragile economic recovery, dismissing critics who argue the policy will lead to future inflation.
"This fear of inflation I think is way overstated," Bernanke said in the interview aired on Sunday.
"What we're doing is lowering interest rates by buying Treasury securities," he said. "And by lowering interest rates, we hope to stimulate the economy to grow faster. The trick is to find the appropriate moment when to begin to unwind this policy. And that's what we're going to do."
Bernanke said it would take four to five years for the country's unemployment rate, which rose to 9.8 per cent in November, to come down to what he called more "normal" levels of around five per cent to six per cent.
Asked if the central bank could go beyond the $600bn of bond buys announced at its November meeting, Bernanke said: "Oh, it's certainly possible. It depends on the efficacy of the program. It depends, on inflation. And finally it depends on how the economy looks," he said.
But he also did not rule out stopping short of the total.
"We're gonna be regularly reviewing this," Bernanke said. "This is not something that we've set into automatic motion going forward. We want to continue to think about it. Whether it needs to be changed. Whether it needs to be increased or decreased or modified."