The current excuse, as the administration has been using or a couple years now, but former President Clinton put so succinctly in his keynote address to the Democratic National Convention this week is, "No president, not me, not any of my predecessors, no one could have fully repaired all the damage [to the economy] that he found in just four years." There are two main problems with this excuse.
First, I don't think the mainstream expectation is that the economy be fully recovered in four years. Everyone knows that the Great Recession was the worst downturn since the Great Depression, and the subsequent recovery would take that much longer. The problem now isn't that we haven't recovered fully, it's that we don't have a significant recovery at all. Normal post-recession recoveries grow GDP at a rate of 3-4%. For the last two years we have only been growing at an average of 2% per quarter.
Unemployment was also worse than we've seen in decades, peaking at 10% in October 2009, four months after the recession officially ended that June. Under normal circumstances it takes anywhere from 12-36 months to return to peak employment levels based on past recessions. Peak employment before the Great Recession was 4.4% unemployment in May 2007. We are now more than five years (63 months) past that point, yet we're still near double that unemployment rate (8.1% for August 2012). At the current rate of job creation (~100,000/month) it will literally take decades for us to return to the near-full employment we had in 2007. So no, we don't expect a full recovery in 4 years, but we expect some significant growth and progress toward that goal. The stagnation of the last three years is insufficient, and makes the President's "proposition" long overdue.
The second reason the Clinton excuse falls flat is the timing of the various economic indicators. The recession began in December 2007 and ended in June 2009. We were already on the way out by the time President Obama took office in January 2009. Unemployment spiked from mid-2008 to the end of 2009, but in terms of job losses, the worst ended just as Obama took office, as Democrats so giddily like to point out in this chart (more accurate version here).
So in terms of both GDP and job losses, we had already hit bottom by the time Obama took office. He took office and proceeded to reassure us over the next year that we would be on the road to recovery soon (the fact that we've had two more summers since the first "Summer of Recovery" exposes that lie). The very fact that he said that he would have it fixed in three years after two weeks in office shows he thought he could, and he already had the information necessary to come to that conclusion.
What they're really saying is that it will take more than 4 years to recover from their errors. The Clinton excuse could be more accurately summed up as, "We didn't know how terribly we'd do." Sorry, that's not how it works, especially when we have no evidence that they've learned from the mistakes of the past 4 years and will take a different approach to fix it now. We have elections every 2, 4, and 6 years so that we the people can decide if you get another shot. On November 6, we'll make that decision for them.