The Fed's most recent forecasts suggest interest rates should stay "exceptionally low" until late 2014. Extending that language to say 2015 or later could spur economic activity by eliminating some uncertainty about future interest and inflation rates.

Economists say such a move is unlikely to have much impact though. So much can change over the next two or three years. The presidential election, for one, could change expectations for future tax and spending policies.

Plus, the makeup of the Fed is scheduled to change. Bernanke's term as chairman ends in January 2014, and whoever his successor is could have different ideas.

Yellen has said "the effects of forward guidance are likely to be weaker the longer the horizon of the guidance, implying that it may be difficult to provide much more stimulus through this channel."

The Fed will release its official statement at 12:30 p.m. ET on Wednesday, followed by a press conference with Bernanke at 2:15 p.m.