Chris Cillizza suggests the answer is somewhere around a half century, as illustrated by a timeline map of presidential and congressional elections, 1960-2008, put together by Cillizza’s WaPo colleagues. Of their work, he writes:

Need a shocker? Compare the map that elected Jimmy Carter president in 1976 — a solidly Democratic south counterbalanced by a solidly Republican west coast — and the one that elected Barack Obama in 2008 — the northeast and west coast for Obama, the plains and south for Sen. John McCain (Ariz.).

Earlier in the same post, Cillizza links to commentary by former National Republican Congressional Committee chair Tom Davis, who suggests a single year is all that’s needed to measure political eternity:

The 2009 off-year elections in Virginia, New Jersey, and New York’s 23d Congressional District offer a small snapshot of the current views and motivations of the American electorate. While there may be a desire to extrapolate the events of Nov. 3, 2009 into a prediction of what will happen on Nov. 2, 2010, that is impossible. A year is an eternity in politics, and there are many variables that will affect the mood of the electorate …

Davis goes on to offer “seven key lessons” from last week’s elections. My favorite of his many points is this “pox on both your houses” claim:

Voter anger is directed against the political establishment, which happens to be dominated by Democrats. However, it would be a mistake to read this as a pro-Republican trend.