Keep it in the primary as long or as short as you want. I do 2 weeks in primary for about 80% of my beers. Then I go straight to bottle for 2 weeks. I generally only secondary for huge beers (>1.080) or those which have fruit added.
Primary fermentation is usually done a few days after pitching. Taking gravity readings is the only way to know if fermentation is complete. The longer you leave it in the primary the more the yeast will clean up the beer. I personally believe that there is no real benefit to keeping smaller beers (<1.060) in fermentation vessels for weeks or months.