Agreed on taking hydrometer readings - 3 identical readings over successive days and you can be confident that fermentation is done. Unless I'm making a wheat beer I rarely keg a beer in under a month. As Jim said, leaving a beer on the yeast a little past the point fermentation stops will let the yeast, in essence, clean up after itself by getting rid of off flavors/aromas that often end up in beer if you rush things. Unless you make a very high OG beer, a fruit beer, or choose to dry hop in secondary (I do), racking to secondary is not necessary, and can be detrimental.