WRT the current economy, we do have one big break. The original crash of the great depression occurred in 1929, and Hoover was in charge until the bottom in 1933. So the economy dropped like a lead sinker for those four years. 

In this case, we have a good liberal on the job relatively early on, though it would have been better to have some adults in charge last year. So look for things to begin to display signs of life later in the year, and then probably sputter through most of next year as well. Hopefully the economy will be moving later in the year of 2010. 

Also we have to remember that in a sense the economy is already getting a certain degree of stimulus from the price of gas. It's almost like a self-regulating mechanism at this point with the world still depending on fossil fuels. As the economy surges, the price of energy surges with it and drags things down somewhat; when the economy lags, the price of energy declines and serves to bring things back.