A lot of people leave their beer in primary for 3-4 weeks.
when you are making a big beer, taking it off the yeast cake prematurely can result in 'green' flavors that persist longer. This is because primary fermentation creates some compounds that contribute to off flavors but which can be metabolized into good old ethanol and co2 by the yeast if you give 'em a chance. In lower-alcohol beers you may not notice. This sort of issue is also decreased by pitching enough yeast.
There's really no reason to secondary at all unless you have specific reasons to do it.